The Web of Life – Plants

Plants

Photosynthesis underwrites most life on Earth. ~ American botanist Karl Niklas

The grouping of plants conventionally includes the progenitor – green algae – and its land plant descendants – embryophytes. On land are mosses, liverworts, ferns, and other seedless plants (pteridophytes), gymnosperms, and angiosperms.

Mosses are small, soft, non-vascular plants. There are 12,000 different mosses: typically, 1–10 cm tall, though a few are larger.

Dawsonia is the largest moss. Found in Oceania, it can reach 65 cm in length.

Liverworts are an evolutionary step from mosses, as their leaves are more developed, and they have rhizoids: root hairs, which are the precursor to roots. There are 9,000 distinct liverworts.

Ferns were the first vascular plant. They have roots, stems, and leaves, but reproduce via spores, not seeds. Ferns first appeared 360 MYA, but the ferns living today date to 145 MYA. There are 12,000 extant species of ferns.

Gymnosperms were the first seed-producing plants (spermatophytes). Conifers, cycads, gnetophytes, and the ginkgo tree are gymnosperms.

Conifers bear their seeds in cones. There are 8 conifer families, 68 genera, and 630 extant species. Conifers include pines, cypresses, and other cone-bearers. A few conifers are shrubs, but most are trees.

Cycads have a stout, woody trunk, topped with a crown of large, stiff, evergreen leaves, which are typically pinnate. Cycads include bread palms, Zamia, and Ceratozamia.

Gnetophytes differ from other gymnosperms in having the water-conducting tissue found in all flowering plants. There are 3 genera of gnetophyte: Gnetum, Welwitschia, and Epherdra, totaling 70 species.

The Gnetum genus comprises ~35 species. Most Gnetum are woody climbers in tropical forests. The best-known Gnetum is the melinjo tree; endemic to Southeast Asia and western Pacific Ocean islands. Melinjo seeds, fruit, flowers, and leaves are used in Indonesian cuisine.

Welwitschia has a single species: an odd, large, but low-to-the-ground plant, native to the Nimib desert on the west coast of Africa. Welwitschia may live 2,000 years or more.

The Welwitschia plant has 2 strap-like leaves that grow continuously from the plant’s base, reaching 2–4 meters. Over time the 2 leaves become flayed by various events, giving the appearance of multiple straps.

Welwitschia have separate female and male plants (dioecious). Insects fertilize. Females produce seed-bearing cones.

The 50 species of Epherdra are shrubs adapted to aridity; found in southwest North America, the west coast of South America, north Africa, and temperate latitudes from Spain to China.

Epherdra have been medicinally employed for at least 60,000 years; treating asthma, hay fever, and the common cold. They contain the alkaloids ephedrine and pseudoephedrine. These compounds – chemically related to amphetamines – have both stimulant and decongestant properties.

Angiosperms enfold the latest innovations in plant technology, most notably beautiful blossoms; a sexual display of unmatched loveliness. There are 352,000 species of flowering plants.

All told, there are nearly 400,000 plant species. Over 90% are seed plants.

A plant and its environment are inextricably linked into one holistic structure. Plants are not passive entities at the mercy of any environmental perturbation, but to an extent manipulating the environment to their benefit. ~ Anthony Trewavas

Plants provide much of Earth’s breathable oxygen. Plants are the macroscopic foundation of terrestrial ecology. All land animals ultimately depend upon plants for survival.

Most plants have 3 major organs: roots, stems, and leaves. Angiosperms add flowers.

Tissues

Each plant organ is composed of tissues: groups of cells performing a function. A tissue is classified by its origin, function, and structure. Any plant organ may have several tissues.

There are numerous tissues in a plant, some simple: comprising a single cell type, such as ground tissue and various fibers; and some complex: with multiple cell types, including vascular and outer tissues. All plant tissues are produced from meristem.

Unlike animal cells, which move during development, plant cells are immotile. As such, the decision-making, hormonal, and genetic networks for plant tissue growth are unique.

Meristem

Plant growth begins with meristem: undifferentiated cells which specialize according to need. Meristematic cells are analogous to stem cells in animals.

The growing tips of buds, shoots, and roots are apical meristem. They provide primary growth.

Whereas primary growth is growth from cell division at the tips of roots and stems, causing elongation, secondary growth is cell division in cambia or lateral meristems, causing roots and stems to thicken.

As a shoot tip extends, stem cells at the center of meristem divide and proliferate. Cells on the periphery differentiate to form plant organs, such as leaves and flowers.

In-between the 2 layers are a group of small boundary cells that become quiescent, forming a barrier that separates stem cells from differentiating cells, eventuating into the borders that define plant organs. Brassinosteroid hormones help establish organ boundaries.

When it is time to flower, shoot apical meristem transforms into inflorescence meristem, from which different flower parts grow.

Apical meristem differentiates into one of 3 primary meristems: procambium, ground meristem, and protoderm.

At the center of a growing root or stem is procambium, which develops into fluid transport (vascular) tissues.

Outside of procambium lies ground meristem, which produces ground tissue of various sorts, including secondary tissues. Ground tissues serve various functions, including leaf energy production (photosynthesis), structural support, storage, secretion, and wound repair.

Protoderm is on the outside of a stem, developing into the epidermis (outer tissue layer).

Secondary Tissues

Secondary growth comes from lateral meristems, which produces secondary tissues that increase the girth of roots and stems. Cambium tissues provide structural support (cork cambium) and fluid conduction (vascular cambium).

Vascular cambium produces vascular tissues. This gives rise to wood in arborescent plants which sustain themselves aboveground throughout the seasons. In contrast, herbaceous plants have leaves and stems that die down to the soil level at the end of the growing season.

Cork cambium, like vascular cambium, forms a thin cylinder that runs the length of roots and stems of woody plants. This protective layer lies just outside vascular cambium.

The cytoplasm of living cork cambium cells secretes suberin: a waxy, fatty substance that renders cork cells waterproof. This protects the tissues within from drying out, freezing, and mechanical injury.

In woody plants, after early growth, the initial epidermis is sloughed off, and replaced by a periderm from tissues produced by cork cambium. Periderm is a secondary covering on small woody stems and non-woody plants.

Within periderm are lenticels: airy cells that act as pores, providing gas exchange between internal tissues and the atmosphere. Fissures in tree bark have lenticels at their bases.

Bark is the outermost layer of the stems and roots of woody plants. Bark is a nontechnical term for the various tissues outside the vascular cambium. On older stems, inner bark is living tissue, while outer bark is dead tissue.

Physics

The physics of plants often approach perfection, geometrically and otherwise. Phyllotaxy is exemplary.

Phyllotaxis is the arrangement of specific plant parts, such as leaves on a plant stem. The phyllotactic opposites and spirals that plants create with their branches, leaves, and compound flowers are genetically imbued patterns, epigenetically adjusted to account for the local situation.

Plant growth is optimized to take advantage of current conditions. Roots spread in patterns to optimize their systemic probability of nutrient uptake.

Leaves grow in a concerted manner to afford each other sunlight, while providing the entire plant maximum solar energy intake. Suboptimal leaves have their contents recycled, leaving a cellular husk that falls away. This phenomenon is seen by trees abandoning their lower branches as they compete with neighbors for sunlight.

Water and light are the energy sources for plant life. But light provides more than energy. Light is also an environmental signal: telling a plant the date of the year, which guides plant growth via hormonal changes. Seeds underground respond to heat signatures generated by sunlight.

Plant growth commonly creates optimal configurations. These patterns often correspond with mathematical properties.

Leaves typically sprout from a stem at an angle that provides the best chance of maximizing nutrient intake, including air, moisture, and sunlight. In beech and hazel, the angle is 1/3; oak and apricot = 2/5; almond and willow = 5/13; pear, poplar, and sunflower = 3/8.

 Sunflower Florets

At the center of a sunflower or daisy is a mesh spiral of florets. The florets mature into seeds.

To give each floret an equal chance at propagating the plant, a specific arrangement must be made. In sunflowers, as in all plants, the growth pattern is geometrically precise.

Mathematically, floral disc arrangement occurs as Fibonacci numbers, as the angle of succession (divergence) – that is, the angle at which the next floret appears in relation to the one before – approaches the golden angle. Fibonacci numbers recur in many organic patterns.

Each floret is oriented toward the next by 137.5°, thus producing an optimal packing of florets. This arrangement is most efficient, both in internal dynamics related to floret development, and in equalizing the opportunity of individual florets to process sunlight, moisture, and air.

Photosynthesis

The photosynthetic system of plants is Nature’s most elaborate nanoscale biological machine. ~ Indian molecular biologist K.V. Lakshmi

Photosynthesis uses the molecular machinery in plant leaf cells to extract from water and carbon dioxide the electrons and protons needed to produce food and fuel. Like all biochemical reactions, photosynthesis is powered by the actions of electrons.

Plant leaves are green because they are filled with chlorophyll: pigment molecules that selectively absorb light spectra, rejecting (emitting) a certain range seen as green. An incoming photon energizes an electron in the chlorophyll into a mobile state.

Once excited, the electron quickly shuttles from the chlorophyll to a nearby acceptor molecule, setting off a series of electron transfers. Photon capture and conversion into an electron sent on its way happens within 10 to 100 picoseconds. A picosecond is one-trillionth (10–12) of a second.

Moving from one molecule to another within a plant cell’s membrane, a transferred electron ultimately reaches a reaction center, where electrical energy is converted into chemical energy: an ATP molecule.

The initial, near-instantaneous electron transfers are incredibly efficient. Over 95% of light energy hitting a leaf reaches the photosynthesis reaction center.

Succeeding biochemical steps are much less efficient. Nutrients directly affect photosynthetic performance, notably the instant availability of nitrogen and phosphorus. The overall metabolic environment also has an effect.

The superefficient energetics of photosynthetic electron transfer occurs using quantum coherence: photons acting as waves, simultaneously sampling potential pathways and instantaneously selecting the most efficient path. This miracle of productive economy is known as Fermat’s principle.

The structural design for photosynthetic organisms is highly efficient. Light-absorbing chlorophyll molecules in leaves are tightly packed into tiny organelles – chloroplasts – crammed into an arrangement where they come into frequent contact. When excited by photonic energy, chlorophylls no longer act as individual cells, but in concert.

Photosynthesis is a collaborative action. Multicellular cooperation allows plants to absorb energy in a wide spectrum of light. In such a system, other light-absorbing pigment molecules, such as carotenoids, transfer energy efficiently. The individual electrons behave coherently, coordinating their movements as they jostle energy through the system.

 Chlorophyll

There are 5 known types of chlorophyll: a,b,c,d, and f. These vary slightly in chemical structure, and functionally by the spectral wavelengths of light that they absorb. Various cyanobacteria use different chlorophyll types; often a mixture.

Chlorophyll a is predominant, absorbing blue light, with a peak at 465 nm, and reddish light, with maximum absorption at 665 nm. Plants are green because chlorophyll a largely reflects green light.

Chlorophylls b, c, and d are shifted somewhat in their peak absorption compared to a. Fewer plants have these chlorophylls. Those that do often employ a mixture of chlorophyll types.

Chlorophyll d and f are found in cyanobacteria. d absorbs the most light at 697 nm. f is the most red-shifted, with maximal absorption is at 722 nm. There is no chlorophyll e.

Chlorophyll, hemoglobin, and myoglobin molecules all have a similar chemical structure. Chemistry efficiencies result in repeated usage patterns in biology.

Hemoglobin is the oxygen transport molecule in animal blood. Myoglobin is hemoglobin’s muscle tissue equivalent. While the atom at the center of the chlorophyll molecular ring is magnesium, the globins use iron.

 Chlorophyll’s Color

Green plants are not as efficient as they conceivably could be. In reflecting green wavelength light, and thus appearing green, chlorophyll seems to miss an opportunity: to absorb as much light as possible.

If it did so, chlorophyll would be black; giving nothing up in reflection. Such efficiency would cast the natural world in an unseemly oppressiveness; vegetation as the death of light rather than its vibrantly verdant employer. As Nature wants to put on a vivid show, vegetation is verdurous.

Unlike the cyanobacteria that evolved chlorophyll, archaea employed a different light-absorbing chemical: retinal, a form of vitamin A, bound to proteins called opsins. Opsins are the photoreceptors of animal vision, and most sensitive to the green portion of the light spectrum: a perfect receiving complement to chlorophyll’s emission.

Archaeal retinal is tucked into the protein bacteriorhodopsin, which appears purple, as it is most efficient in absorbing green light, peaking at 568 nm. Bacteriorhodopsin translates light energy to biochemical form via a proton pump: pushing a proton across a cellular membrane.

Systemically, bacteriorhodopsin’s efficiency is less than that of photosynthesis. The photochemical cycling time for retinal is slower. While photosynthesis ultimately pushes a proton, it happens later in the process.

Retinal-based archaea may have dominated the waters of early Earth. Retinal has a simpler structure than chlorophyll and would have been easier to produce in the low-oxygen environment of the eon.

These flagellate archaea swim about, optimizing light exposure: neither too bright nor dim. Bacteriorhodopsin acts as both as an energy source and a receptor.

Bacteria trying to competitively sunbathe below archaeal mats managed by picking up on other wavelengths. Hence the evolution of more efficient chlorophyll, with absorption peaks outside retinal’s optimal band. As it turned out, 2nd-generation photon absorbers became dominant.

The core machinery for photosynthesis is highly conserved genetically, having been optimized over 3.5 billion years ago. Pigment and protein organization and interaction is the same in cyanobacteria and plants.

The specific light-harvesting complexes in plants evolved later, shaped by adaptation to the ecological niche in which a particular plant lives.

Evolution is not a start-from-scratch engineering exercise. It instead involves ongoing adaptation based upon extant architectures and existing constraints. That both biochemical energy procurements from light – via retinal and chlorophyll – transpire with such efficiencies is an impressive statement of coherence in action, especially considering they evolved independently.

Water

Water constitutes nearly 90% of the growing tissues of plants, and 5–15% of seeds. Managing water is a critical challenge.

Water is the medium for plant biochemical reactions and transport. Water’s peculiar properties, including polarity, solvency, and viscosity, largely determine how plants are structured, as well as their metabolic dynamics.

Most biochemically important compounds are charged. Water polarity facilitates essential biochemical reactions, while its solvency tempers some reactions, such as the attraction of sodium (Na+) and chloride (Cl). While it may seem obvious that all cells take advantage of these water properties, the subtle attunement of life to the physics of chemistry remains a marvel.

The nature of the hydrogen–oxygen bond accounts for the high tensile strength of water in plant capillaries and enables water to exist as a continuum in plants: through roots, stems, and leaves. Properly managed, water easily flows.

Water’s resistance to pressure makes it a useful hydroskeleton. Land plant leaves owe their rigidity to the water pressure inside them, which is commonly 3–10 atmospheres (0.3–1 MPa). This pressure is vital during cell expansion. Plant tissues move by carefully altering relative pressures within.

Some water properties seem less useful. The rates at which oxygen and carbon dioxide diffuse in water are very low: 10,000 times slower than in air. This may have imposed a limit on the maximum thickness of leaves. To compensate, vascular plants evolved intercellular spaces in leaf tissue so that these gases need not diffuse in water by more than the length of a single cell.

Water’s relatively high viscosity creates considerable tension during transport, especially transpiration, when water is drawn through the fine capillaries of cellulose cells walls and xylem. But then, plant tissues are designed precisely to account for this level of viscosity and employ those specific properties to advantage.

Transport

Land plants (embryophytes) evolved from aquatic plants, such as green algae. The earliest embryophytes were small and simple, with a limited nutrient distribution system. These are bryophytes: mosses, hornworts, and liverworts.

Then came a great innovation: a tubular transport system to shuttle water and nutrients around. These are vascular plants (tracheophytes). A tubular distribution system afforded great versatility in plant morphology.

The art of evolution is evidenced by traits that later find more sophisticated employment, and in species that show the first steps to later developments. Some bryophytes have specialized tissues to transport water, but they lack lignin. Red alga, one of the oldest algae groups, has lignin. Tracheophytes emerged by putting the 2 traits together.

 Xylem & Phloem

Long-distant transport sustains life in multicellular organisms. ~ Japanese botanist Kaori Miyashima Furuta et al

Animals transport nutrients between cells. In contrast, plant transport networks are through cells.

Instead of generating specialized extracellular spaces, lined by cells, as in the case of blood vessels, plants have come up with a way to generate a transport route within the transport cells themselves. ~ Swiss botanist Niko Geldner

There are 2 types of plant vascular tissue: xylem and phloem. The distribution processes of both are optimal, limited only by the constraints imposed by physics.

Xylem is the major long-distance transport system, of water and nutrients from roots toward the upper portions of a plant. Wood is the best-known xylem tissue.

The explanation for easily defying gravity to effect the ascent of sap is termed the cohesion-transport theory, developed by several dozen scientists over the course of a century.

The xylem vascular pathway has hydraulic continuity: integrated variations in requisite cellular (tissue) pressures throughout the plant, allowing instantaneous transmission as needed, based upon surface tensions at evaporating surfaces, which are ultimately dependent upon the tensile strength of water.

In trees, xylem is next to the pith, while phloem is outside xylem, as the innermost layer of bark. Pith is the soft, spongy growth tissue inside a plant.

Phloem carries nutrients made during photosynthesis (photosynthate), in a process termed translocation. Photosynthesis-derived sugar travels from sugar sources to sugar sinks by means of a turgor pressure gradient: cellular osmotic flow.

During growth, roots are sugar sources and a plant’s many growing areas are sugar sinks. After the growth period, when meristems become dormant, leaves are sources, and storage organs are sinks. Seed-bearing organs, such as fruit, are always sugar sinks.

Unlike xylem’s upward flow, phloem translocation is multidirectional.

Another contrast comes in the state of cells involved. Xylem consists primarily of dead cells. Phloem comprises a complex of coordinated living cells.

Phloem is formed by an interconnection of sieve element cells that stretch from the roots to tender leaflets. Sieve elements fortify their cell walls, get rid of their nucleus and most of their organelles, and generate a highly fluid cytosol that allows for flow rates of up to 110 µm/s. Cytosol is cytoplasmic fluid.

Sieve elements depend upon companion cells to which they are intimately connected. Thus, phloem sieve element cells impair their own ability to maintain cellular metabolism, but remain alive thanks to sustenance from their neighbors, whose extensions they effectively become.

Phloem differentiation challenges our basic concepts of cells as functional units and our definition of what constitutes a living cell. ~ Niko Geldner

The distinction between deceased xylem and living phloem cells lies in the nature of distribution regulation. Xylem water transport is mostly a matter of evaporative physics, whereas phloem translocation is an exquisite chemical process guided by physics. Whereas xylem water movement is driven by negative pressures (tension), phloem translocation operates via positive hydrostatic pressure: pressure exerted by a fluid at equilibrium due to the force of gravity. Though the physics differ, functionally, the phloem and xylem transport networks are selfsame in delivering nutrients when and where needed.

Size Limits

The advantage to large leaves is greater area upon which sugar-producing photosynthesis might be performed. With their sturdy construction, massive angiosperm trees could easily hold large leaves.

But all tall trees have rather small leaves. While shrubs and short trees may have leaves anywhere from less than 2.5 cm to well over a meter, the tallest trees all have leaves only 10–20 cm long.

Both the size of leaves and the height of trees owe to limits imposed by fluid dynamics. On a tall tree, nutrient fluid flow would be too slow with leaves too small or large.

There comes a point where the optimal limits on leaf size and tree height intersect, indicating the point at which it is no longer advantageous for the tree to become taller or produce larger leaves. ~ Polish botanist Maciej Zwieniecki

Conifers have the tallest trees, notably the coast redwood, which may reach 115 meters high. The clustered needles of these trees differ depending upon how high up the tree they are.

Exposed to more dry heat, treetop needles are tight spikes that conserve moisture by having little evaporative surface. In contrast, needles on lower branches have larger, flat needles that catch more light coming through the thick canopy. This is one instance of many where trees optimize their leaves to fit the situation.

Chemistry

Plants use complex metabolic pathways to fend off pathogens, to coordinate reproduction with changes in day length, to accommodate environmental changes, and to select developmental pathways most suited to a given place and time. ~ American biologist Pamela Hines & American botanist Laura Zahn

Plants are unsurpassed chemists. DNA, sugars, starches, proteins, and oils are all constructed on carbon backbones, bonded to form complex compounds. Unlike animals, which produce these substances autonomically, plants are consciously involved.

Metabolites are the products of plant metabolism. Metabolites are classified by perceived essentiality: primary and secondary.

Primary metabolites are the necessary chemical products for energy, construction, and reproduction. Without the requisite materials to produce primary metabolites, a plant quickly dies.

Secondary metabolites are specialty compounds, commonly concocted to defend against herbivory, from creatures large and small. Certain secondary metabolites are somewhat more constructive: carving exclusive territories under competitive conditions, by inhibiting germination and growth of other species, and creating conducive conditions for favored allies, notably microbes and pollinators.

Unlike primary metabolites, secondary metabolites are not essential to a plant’s immediate health.

Plants do not produce all their metabolites in every cell. Tissue-specific factories synthesize many metabolites, especially secondary metabolites, as these are typically hazardous materials.

 Vacuoles

Most mature plant cells have a central vacuole that houses the primary plumbing and several cell workshops. Vacuoles typically take 30% of a cell’s volume and may occupy as much as 80% of the volume, depending upon cell type and condition.

Cytoplasm strands commonly run through a vacuole. Gel-like cytoplasm carries a cell’s internal sub-structures.

Vacuoles store sugars and host the secondary compounds that variously perform to plant needs as a matter of ecology. Vacuoles are a central setting for the metabolic show that plant cells put on.

Primary Metabolites

Powering, patterning, and perpetuating are the 3 essential needs of all life: the energy to keep going, the materials that make and maintain an organism, and the ability to propagate. They all come down to chemistry.

Biology is a complex chemical dynamic with a vital sprinkle of life-energy (lengyre). That lesson is perhaps easiest and most convincing to see in plants, as they possess such beauty and elegance in their chemical concoctions, and because of their essentiality to other multicellular life.

Carbohydrates

Photosynthesis in plants leads to the accumulation of carbohydrates (e.g., sugars, starch), upon which all terrestrial life depends. ~ American botanist David Braun

Carbohydrates all adhere to the chemical formula: Cm(H2O)n: carbon and water, where m and n differ. The term carbohydrate is a misnomer, as carbohydrates are not actually hydrates of carbon.

Carbohydrates’ different forms are all of fuel storage, from the simplest sugars: monosaccharides, which are easily digested, to complex starches: polysaccharides. Plant gums are a viscous polysaccharide.

The m in monosaccharides runs from 3; in polysaccharides, m is between 200 to 2,500.

Glucose is a simple sugar, one of the main products of photosynthesis, the starter of cellular respiration, and a primary energy source. Glucose is a ubiquitous organic fuel, for bacteria and humans alike.

Via aerobic respiration, glucose is the human body’s key source of energy. Starches break down into glucose, which is oxidized to eventuate into carbon dioxide and water as byproducts. ATP is the goal: carbon, hydrogen, oxygen, with a dash of nitrogen and phosphorus.

Beside pure energy, several polysaccharides play important roles in the activation of animal immune systems.

Lipids

Lipids are a broad group, affording various forms of energy storage, though with other important biological roles as well. Fats, waxes, sterols (e.g., cholesterol), and fat-soluble vitamins (A, D, E, K) are exemplary lipids.

Lipids are used to make cell membranes, of both plants and animals. Lipids also play a vital role of cell signaling.

In a process termed lipogenesis, an oversupply of carbohydrate are converted to triglycerides for energy storage. Conversely, these fatty acids are broken down for energy via beta oxidation: a similar, though not identical, reversal of lipogenesis.

Plants stuff their seeds with carbs, lipids, and proteins before sending them on their way to make a life for themselves. They also pack in a beneficial microbiome, to make sure a seedling starts out among friends.

For those that don’t make it to sprouting, seeds provide a rich nutrient source for animals.

Amino Acids

All life relies upon amino acids to render a vast array of proteins, which are the molecular actors of biological existence. Besides meeting the needs of development, growth, and reproduction, plants combine proteins with secondary metabolites for self-protection.

For over 300 million years, plants have been the primary producers of ready-made proteins and amino acids for animal consumption.

Secondary Metabolites

Plants are very sophisticated chemical factories; able to produce thousands of different compounds, each one presenting unique biological properties. ~ Swedish botanist Stefano Papazian

Though impressive works of molecular engineering, the fundamental cellular compounds common to all green plants – DNA, chlorophyll, and cellulose – are relatively simple. In contrast, plants’ molecular wizardry is dazzlingly demonstrated by their secondary compounds, which are strikingly sophisticated. Over 80% of the organic compounds in Nature come from plants. Plants produce over 200,000 distinct chemical structures.

Secondary metabolites are specialties, produced for defensive purposes, concocted by adding specific elements to carbon rings. These metabolites illustrate a plant’s evolutionary lineage.

Plants evolved a vast diversity of defensive metabolites. This diversification aimed at novelty – to develop a compound to which an herbivore had not adapted to digest, thus putting the predator off. Plant preference for secondary metabolite production has been a factor affecting floral speciation.

Tannins become bitter via highly reactive hydrogen and oxygen. Terpenes too, in a more complex set of carbon rings.

Alkaloids nestle nitrogen. Sulfur succors mustard oils, and infuses the defenses of onions and garlic. Glycosides are a sugar, with a toxic sprig attached.

Producing secondary metabolites come at a cost to the plants that produce them. It takes energy and raw materials to construct these organic molecules, as well as investment in complex biochemical pathways.

Plants alter production and storage of secondary metabolites as needed. Individual plant organs modulate their employment of compounds and communicate their status and anticipated material requirements to those plant parts which produce or secure resources. Plants learn from experience and better strategically regulate metabolite production.

Flavonoids

Judge a tree from its fruit, not from its leaves. ~ 5th century bce Greek playwright Euripides

UV radiation can mutate DNA. Early on, as a protective shield, plants fabricated flavonoid pigments which reflect UV.

Flavonoids were later put to a variety of uses. Their diverse employment owes to versatility via structural simplicity: 2 6-carbon rings connected by a 3-carbon ring.

Plants attune the chemical composition and arrangement of flavonoids for graphic employment. Slight changes in one flavonoid group provides the reds, violets, and blues in flower petals. Flower patterns drawn by flavonoids act as billboards to pollinators.

Flavonoids are water-soluble and color-sensitive to acidity, so one molecular structure can display a range of colors depending upon the acidity of the cell vacuole where a flavonoid is housed. Plants comprehend the colorful implications of certain molecules in specific settings.

Flavonols, a flavonoid group, are white to translucent compounds, common in leaves, stems, roots, flowers, and fruits. Converted to anthocyanins, they reflect UV while turning leaves to autumnal colors.

Red leaves may act as camouflage from herbivores, almost all of which have dichromatic vision, and so cannot clearly distinguish red. Or, to a dichromat, the leaves may appear a somewhat distinct yellow, and so instead signal an unpalatable choice, as anthocyanin synthesis often coincides with production of phenols, an acidic molecule like alcohol.

Tannins

Tannins are another ubiquitous secondary metabolite. This bitter, astringent compound can drastically limit a plant’s potential as a nutritional source. It has even been suggested that the extinction of dinosaurs was hastened by tannin poisoning.

Tannins are yellow or brown. They commonly accumulate in the bark of trees. Tannins also play a role in taking care of offspring.

As summer turns to autumn and the nights grow crisp, trees economize, sucking chlorophyll from their leaves for sustenance and substituting tannin, an antimicrobial agent. The leaves turn from green to yellow, and then, filled with Sun-activated tannin, turn a bright red before dropping to the ground around the tree.

The fallen leaves of autumn create a carpet of protection. When seeds fall from the tree, a tree’s seedling offspring have a better chance of survival, the ground being sown so that the tree’s precious progeny have less chance of being eaten by bacteria and fungus.

Mice and deer sometimes die from eating acorns, which are typically high in tannin, though the concentration varies. Red oak acorns have 2 to 4 times the tannin of white oak acorns. Some animals, including certain woodpeckers, squirrels, and mule deer, evolved a high tolerance to tannins without ill effects.

Animals reject food plants with a concentrate of tannin because of the bitter taste, but also because ingestion brings a biochemical kick. If an animal eats tannin-laden leaves, the tannin interferes with metabolism by binding to digestive enzymes, and to otherwise edible proteins in plant tissues. Because tannins are metal ion chelators, they inhibit the absorption of minerals such as iron.

Long-term exposure to tannin compounds present a cancer risk. In Hunan, a province of China, black teas high in tannin are a traditional beverage. Cancer of the esophagus there is relatively frequent. Woodworkers and leather workers, where tannin is used as a softener, are prone to nasal and sinus malignancies.

Tannins provide protection to a plant at considerable cost in energy and resource expenditure. Up to 30% of the dry weight of an otherwise eminently succulent maple leaf comprises tannin molecules, making it indigestible to herbivores.

Terpenes

The pine stays green in winter… wisdom in hardship. ~ proverb

The pungent scent of pine and spruce is of terpene: a hydrogen-carbon compound, made by a process so complicated that its complete synthesis has eluded chemists. The term terpene is derived from the turpentines which are distilled from conifer resin.

Terpene is colloquially called an essential oil; essential in its carrying a distinctive scent, or essence, of the plant from which it is derived. Essential oils do not constitute any distinction for any purpose, whether medical or culinary. The term has fallen out of use by evidence-based researchers for its lack of specificity; as if essence did not exist.

 Conifers

Conifers are cone-bearing trees, with a few shrubs among the 630 extant species. Cedars, cypresses, firs, hemlocks, junipers, kauris, larches, pines, redwoods, spruces, and yews are conifers.

Conifers are the dominant plants in the forests where they reside. The conifer lineage extends at least 300 million years.

Boreal conifers are adapted for winter conditions. Their narrow conical shape and drooping limbs readily shed snow, thus relieving them of an otherwise heavy burden. Many conifers harden for winter by altering their biochemistry to resist freezing.

The tropical rainforests have more biodiversity and turnover of species than boreal forests, and so are celebrated by naturalists as exemplifying Nature’s dynamic beauty. But the extensive conifer forests of the world represent the largest terrestrial carbon sink: binding carbon as organic compounds. Thus, conifers’ part in the planetary carbon cycle confers immense ecological importance. Conifer wood (termed softwood) also has great economic value for wood and paper production.

The sap that runs through the wood and needles of conifers are the first line of defense from animal attack. These terpenes are strong organic pesticides.

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Trees are not the only ones packing terpene pesticide. Nasutitermitinae termite soldiers synthesize terpene and shoot it at enemy insects using a fontanellar gun: a built-in squirt gun triggered by contracting mandibular muscles, which shoots several centimeters. Termites are accurate shooters even though they are blind.

Egyptian mummies were preserved using pine and fir resins: spread in body cavities as well massaged all over the body, including the eyeballs. The proof of their efficacy against insects is in the remarkable state of corpses thousands of years old.

There are numerous terpenes produced by various parties, though plants were terpene pioneers. The aroma and flavor of hops, happily employed in beer, come from terpenes. Vitamin A is a terpene.

Steroids are derivatives of squalene, a terpene commercially derived from shark liver oil as well as vegetable oils. All plants and animals, humans included, produce squalene.

Different terpenes are employed as insect repellents and mate attractors. Perfumes are concocted from essential oils that come from trees, fruits, and flowering plants. The scent of numerous essential oils are mood lifters.

Terpenes have many medicinal uses: treating sore throats, digestive difficulties, wounds, and cancer. Essential oils are essential in folk and alternative medicine, especially aromatherapy. Many are antiseptic.

Essential oils have long made the scene of psychotropic recreation. The terpene tetrahydrocannabinol (THC), found in hemp, is the psychoactive ingredient of cannabis.

Other mind-addling plant products come as alkaloids.

Alkaloids

Opium teaches only one thing, which is that aside from physical suffering, there is nothing real. ~ French novelist André Malraux

The bitter taste of many plants owes to their alkaloids: compounds comprising carbon rings laced with nitrogen, with a hydrogen kicker. Other elements appear in the alkaloid act, including oxygen and sulfur, with cameos by bromine, chlorine, and phosphorus.

Swiss chemist Carl Meissner coined the term alkaloid in 1819; a term derived from its place on the pH scale. Alkaloids are typically alkaline.

There is no clear chemical boundary between alkaloids and other nitrogen-containing organic compounds. The designated group has held together merely by convention.

Like terpenes, alkaloids are a chemically diverse group of compounds. They originate in plant cells from amino acids. Bacteria, fungi, and animals are also alkaloid producers.

Only 20% of angiosperms synthesize alkaloids. More than 10,000 different plant alkaloids have been identified. Many plant species synthesize several. The opium poppy produces certain alkaloids popular since antiquity: morphine, codeine, and 24 more.

19th-century German pharmacist Friedrich Sertürner first isolated morphine from opium; the first alkaloid to be isolated from any plant. He aptly named it after the Greek god of dreams, Morpheus.

The supply of nitrogen to a plant is always limiting, even in plants that can fix their own, such as legumes. As such, alkaloids are always produced in very limited quantities: as little as 10% of comparable phenol production (e.g., flavonoids and tannins). Such low concentration is compensated by potency.

Rather than mere deterrents to animal grazing, as terpenes typically are, alkaloids are serious: toxins that can gravely damage or poison their consumers. Physiological responses range from the jittery alertness of caffeine to muscle paralysis and death by the coniine in hemlock.

Ancient Greek philosopher Socrates was put to death by drinking a brew of hemlock. His offense was corrupting the minds of youth in his native Athens by insistently refuting the popular creed that “might makes right,” among other heresies, and of the impiety in not believing in the gods of the state.

Affection for mind-altering alkaloids is legion for humans, but decidedly bad news for other animals that cannot afford psychedelic recreation. The dramatic disorientation of hallucinogens can be deadly to creatures that constantly rely on their senses to survive: to avoid predation, falling, and other natural hazards.

Glycosides

Glycoside molecules are a sugar bonded to a non-sugar moiety. The moiety is the business end of a glycoside: frequently toxic, designed to be disruptive to attackers of the plant producing the glycoside. During glycosylation, a sugar is added to the moiety so that the toxins will not harm the plant itself.

Glycosides are activated by breaking the moiety off from the sugar via enzyme hydrolysis: an enzyme breaks down the polymer by adding water (hydrolysis). A glycoside may be activated when a plant part is damaged. Herbivores release the moiety toxin in their guts when digestive enzymes (glycosidases) cleave off the sugar.

The rose family has 2,830 species, including herbs, shrubs, and trees. Besides beautiful flowers, there are also many edible fruits in the family: apples, apricots, cherries, peaches, pears, plums, and strawberries, as well as almonds.

The thorns on rose bushes are one thing. But a deeper danger lurks inside many rose family members. Whenever plant tissue is crushed or damaged, a glycoside is activated that creates a cyanide compound: hydrocyanic acid (HCN).

The familiar scent and taste of bitter almonds comes from amygdalin, a glycoside which disrupts the metabolic activity in the mitochondria of cells when digested. The cells can no longer use oxygen and quickly expire.

Cyanide poisoning in humans, and probably other animals, causes dizziness, nausea, vomiting, collapse, and respiratory failure.

The extract of bitter almonds has been used as a remedy for thousands of years: within the pharmacology of ancient Egyptians, Chinese, and Pueblo Indians as a purgative.

In the 1830s, HCN was extracted in purified form, at which time it was labeled amygdalin. Since the early 1950s, modified amygdalin has been marketed as a cancer cure under the name laetrile.

Cyanide poisoning is indiscriminate. Like a rose by any other name, amygdalin/laetrile has no specific effectiveness against tumors.

However hoary its application, amygdalin never had legitimate standing as medicinal. Substances with a bitter taste are almost always a warning of indigestibility.

In contrast, some of the glycosides known as saponins possess medicinal potential. The first known saponin came from soapwort. Early American settlers from Europe brought the seeds of soapwort to grow and use as a soap, and as an effective folk remedy for skin ailments.

Some saponins are antibiotics, having been designed by plants to protect their roots from soil fungi. Other saponins inhibit the growth of human cancers.

An animal will instantly decide on another food source when faced with a mouthful of alkaloid-laced leaf. This time-worn strategy works well.

Inscrutably, some plant species have mastered a most intimate understanding of their predators. Chemical deterrents that alter the life cycle put a profound hex on an herbivore, and so are more controlling, because their legacy affects future generations. This limits predator population over a considerable expanse of time.

Plants produce molecules that mimic the hormones which control animal reproduction and development. They package these poisonous parcels in precise places where a threat may arise: leaves, stems, and/or roots.

Insects follow 1 of 2 development plans: upsizing or stages. Primitive insects grow by shedding their exoskeleton, replacing it with a new one of slightly larger size.

A more sophisticated development path transitions an insect through distinct stages: egg to larva, pupa, and culminating in a sexually mature adult. A butterfly metamorphizes to an adult by a debut as a caterpillar that heads into a cocoon stage to develop as a pupa.

Having learned the key to both insect development plans, plants produce – as part of a glycoside – a mimic of the specific hormones necessary to deter development. Balsam fir produces a glycoside that keep young insects in a juvenile state, unable to reproduce.

Some plants produce precocenes: compounds that inhibit juvenile hormone biosynthesis. Certain precocenes can accelerate development and metamorphosis, causing small, deformed adult insects that cannot function. Other precocenes induce sterility in some insects, or dupe an insect into diapause, a form of dormancy. This induced diapause effectively shutters insect development by lowering metabolic rate.

Many plants produce steroids that are more potent than the hormones that the insects themselves synthesize. The bugleweed is often the last remaining plant after locusts swarm over large swaths of Kenyan savanna. That’s because the bugleweed produces a hormone that produces developmental defects that deal death to the offspring of an insect that dares eat it. Ravenous locusts know better than to chow down on bugleweed.

Steroid-based glycosides from Mexican yams work as oral contraceptives in humans: stimulating hormone levels so that ovulation and fertilization are precluded. Various plants ply this trick. Australian sheep become infertile if they graze on wildflowers that synthesize hormones that mimic estrogen.

In the western United States, vole populations plummet during late summer and fall as they graze on grasses that synthesize sizeable quantities of phytoestrogens at the end of the growing season. The phytoestrogens retard vole reproduction, right at the time when the mountain grasses are coming out with a new crop of seeds that need protection from voracious voles.

By their effects, glycosides reveal a deep understanding by plants of the functional physiology of animals. Various glycosides affect the beating of the heart, creating cardiac conniptions.

Soybeans and other legumes produce phytoestrogens. These are not as potent as animal estrogens or plant steroids. That’s because they are 3-ring flavonoids rather than 4-ring steroids.

These phytoestrogens may have been an effective deterrent to herbivores at one time, but evolution has provided an edge to the consumers. For humans who regularly consume these fruits, these compounds help lower rates of osteoporosis and certain cancers.

Plant Mechanics

Plants are as adept with molecular engineering for structural attributes as they are for chemical effects. Depending upon what cell walls are made of, and how their layers are arranged, plant tissue can be flimsy or sturdy, a growing tip or a solid wall of protection.

Cell walls are secreted by the living protoplast of a plant cell, outside the membrane that encloses the protoplast. A cell wall has distinct layers, sequentially secreted, with the oldest layer furthest out, and the youngest layer neighboring the plasma membrane.

Materials

There are 4 main materials for plant cell walls: cellulose, hemicellulose, lignin, and pectin. Plants employ them adeptly.

Cellulose is the main structural fiber in plants, and in many algae. 1/3rd of plant matter is cellulose. Some bacteria secrete cellulose to form biofilms.

Hemicellulose is quite a contrast. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has an amorphous structure, with little strength: 40 times weaker than cellulose, and 60 times less stiff. Hemicellulose is readily hydrolyzed.

Lignin is a complex polymer, used to fill the spaces in cell walls, notably xylem, as well as ground tissue (parenchyma cells). Lignin and hemicellulose have similar strength and stiffness properties.

Lignin plays a critical role in conducting water in plant stems, as it is hydrophobic. Its water-repellent property allows vascular tissue to transport water efficiently, especially considering that the polysaccharide components of cell walls – cellulose, hemicellulose, and pectin – are highly hydrophilic.

Lignin is significant in the carbon cycle, as it sequesters atmospheric carbon into vegetative tissue, and is one of the slowest plant materials to decompose. Hence lignin is a major part of humus: plant matter that will naturally decay no further, and may remain as is for centuries, if not millennia. Humus considerably improves soil structure by contributing to moisture and nutrient retention.

Pectin helps bind cells together and allows cell wall extension. Thus, pectin is instrumental in plant growth. Pectin is found in the cell walls of soft, non-woody tissue, such as fruit.

Cell Growth

In nutrient-rich conditions, plants synthesize many proteins and lipids to divide and grow quickly. Under nutrient-low conditions, they stop this synthesis and elongate their bodies to digest unnecessary organelles. ~ Japanese botanist Kiminori Toyooka

The cell walls of plants are comprised of cellulose fibers reinforcing a matrix of hemicellulose, along with lignin or pectin in one or more layers, with the relative volume and orientation of the cellulose fibers varying in each layer.

As a cell grows, the primary cell wall layer is secreted first, comprising cellulose fibers in a matrix of hemicellulose and pectin. Hemicellulose binds to the surface of the cellulose microfibrils, while pectin cross-links the hemicellulose of adjacent microfibrils.

During cell growth, enzymes keep the cell wall pliable. Organelles within a cell proliferate and move about to meet processing demands.

Once growth is complete, cell wall stiffness and strength typically increase. The number and size of organelles decrease.

In some plant materials, notably wood and palms, additional secondary layers are deposited, comprising cellulose fibers in a matrix of hemicellulose and lignin. Cellulose fibers are typically oriented at different angles in each secondary layer.

Layers may differ in thickness. Plants expertly architect cell walls at the molecular level on up for specific application.

Substituting lignin for pectin in secondary layers strengthens and stiffens the cell wall. In mature cells, such as xylem, the protoplast dies, leaving the cell walls to provide mechanical support, while the lumen (empty space) allows water and liquefied nutrient transport. With age, the middle cell wall layers and primary cell wall may become lignified.

Properties

Strength and pliability/stiffness come from an intricate combination of cellular microstructures: cell wall material composition, the spatial dimensions of the cell wall, the number of cell wall layers, cell density distribution, and the arrangement of cellulose fibers in the layers. Fibrils of cells are precisely arranged on a layered basis to achieve the desired properties, which can vary widely.

Parenchyma

Parenchyma is the most common and versatile ground tissue in plants, found in almost all major parts of higher plants. Parenchyma has thin and flexible cell walls, with only a primary layer, comprised of riotously arranged cellulose fibers reinforcing a matrix of hemicellulose, pectin, and glycoproteins. Parenchyma cells have no lignin.

When first produced, parenchyma cells are spherical. But when packed together, their arrangement becomes polygonal, commonly with 14 sides.

Mature parenchyma cells can divide long after being produced by meristem. When a cutting (stem segment) is induced to grow, it is the parenchyma cells that begin dividing and give rise to new roots. When a plant is wounded, the ability of mature parenchyma to multiply is particularly important to tissue repair.

Parenchyma tend to have large vacuoles, which store various secretions, including grains of starch, oils, and secondary metabolites.

Cells which contain chloroplasts, such as leaves, are collectively termed chlorenchyma tissue. Chlorenchyma primarily performs photosynthesis, whereas other parenchyma mostly store food and water.

A transfer cell is a parenchyma cell that is stretched so that the surface area of the plasma membrane is greatly increased. Transfer cells deliver nectar in flowers. In carnivorous plants, they transfer dissolved matter between cells.

Parenchyma cells can have a long life. For instance, cacti cells may live over a century.

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The parenchyma tissues that comprise fruits and vegetables are the least stiff, and at the low end of tensile strength. In contrast, the wood of desert palms is made of cells that are 1,000 times as strong and 100,000 times stiffer.

Wood cells are organized in a honeycomb pattern. Cell walls comprise a large portion of the cell; hence wood’s stiffness and strength.

Woody trees grow in diameter over time. In contrast, palm trees, such as coconut trees, maintain a diameter throughout life.

As a palm stem grows in height, the thickness of cell walls increases to support the extra weight. Cell walls are thickest at the base and periphery of stems, where bending stresses are the greatest.

Flowering Plants

If we could see the miracle of a single flower clearly, our whole life would change. ~ Buddha

Flowering plants selectively transport macromolecular traffic, such as proteins and RNA, between cells through tight, regulated channels called plasmodesmata. Plasmodesmata channels are so narrow that proteins, with an assist from transporter molecules, unfold for passage through a channel, then fold back up at the destination cell.

Within a meristem, functional components are especially steered to specific destinations to orchestrate stem cell division and specialized tissue development. Plants move regulatory bits from cell to cell, often over long distances, to control various activities, including growth and flowering.

Florigen is a signaling molecule that initiates flowering. Prompted by environmental stimulus, leaves produce florigen. Florigen moves into the phloem stream, then travels up the stem to the vegetative apex, where it reprograms the growing tip (shoot apical meristem) to produce flowers rather than leaves.

The communication network of a plant is vast, and the informational and regulatory molecules diverse. Transport is highly regulated, and destination specific. Responses to changing environmental conditions, such as the flow of florigen, are an intricate orchestration.

Pollination

Will the wind ever remember? ~ American musician Jimi Hendrix in the song “The Wind Cries Mary” (1967)

In most places, the wind is an unreliable partner. Animals may be more dependable. But enticing animals willing to work for wages requires advertising.

However free the breeze, plants had been long used to producing compounds and altering their forms to deal with animals, particularly putting off herbivores. Putting energy into attracting animals to aid a plant’s prolific propagation seemed a fair exchange.

Pollination systems reflect a biological market. Potential pollinators may choose their visits based upon advertised allures and rewards.

Plants are careful to honestly advertise their rewards; which is why they signal that a pollination visit has recently occurred, and so landing upon a certain flower is not worthwhile in the moment, as the flower is being restocked. Because attracting pollinators is a competitive exercise, floral deceivers that rely upon pollination do not exist, as plants understand the social dynamic involved.

Insects are the most common pollinators. Angiosperm-insect coevolution occurred among many species of both plants and animals.

Having suffered their herbivory, plants already knew insects intimately, so coopting them was not so difficult. Pollinating bees evolved from wasps during the rapid diversification of flowering plants during the early Cretaceous.

Such luring was a trick learned much earlier. Primitive mosses emit smells that attract tiny arthropods – springtails – that inadvertently pick up sperm and cart it to another moss.

At one time, pollen itself was an attractive enough food. But then other plants did the same, demonstrating the limit of pollen packing as the only ploy to attract pollinators.

Competition being what it is, signage became more flagrant and fragrant. Flowers began to ostentatiously advertise: by their shapes, patterns, colors, and scents. This explains the enormous diversity in the florid displays that angiosperms put on to bewitch pollinators.

Nectar was a logical next step: a sweet treat to attract bees on the beat. Soon, thanks to a competitive market, something more than sheer sugar-water was needed to keep pollinators coming back. Remembering floral traits is difficult when flickering from flower to flower at a fast pace. A memory aid would help an angiosperm’s cause.

Many plants produce alkaloids to ward off herbivores by their bitter taste and psychoactive effects. Caffeine and nicotine are exemplary.

Caffeine evolved independently in tea, coffee, and cocoa plants; put in leaves or seeds to discourage herbivory. But carefully applied, caffeine has a power beyond dissuasion.

Some flowering plants, including coffee and citrus, lace their nectar with caffeine. The bitterness should be off-putting, but just enough is added by plants to have the intended effect while preserving good taste.

Caffeinated nectar provides a tremendous memory boost to a visiting pollinating insect. A little caffeine buzz has a bee remember a specific sweet spot by its floral scent for days. By enhancing a pollinator’s memory, plants reap reproductive benefits through pollinator fidelity.

The effect of caffeine is akin to drugging, where bees are tricked into valuing the forage as a higher quality than it really is. ~ Swiss ethologist Roger Schürch

Plants further help their pollinators by putting in metabolites which prevent parasites and pathogens. Plants take prodigious care of those who take care of them, even if inadvertently.

Competition for pollinator attention cannot be understated. Many bees collect honeydew produced by scale insects found on shrubs. Though relatively unadvertised, the wafting scent of sugar, and social cues from other foraging bees, is enough to attract insects with a taste for sweets.

Plants made the acquaintance of birds as both benefactor – by eating plant pests – and as seed stealers. Induced by florid flowers and fruits, birds got into the pollination business.

For fruit eaters, plants have a follow-on trick for quick seed dispersal: adding a laxative well-suited to its consumers. For plants, sugars are easy to produce. But let’s not have potential offspring sit in the guts of nasty animals, where fermentation might doom the next generation; hence the laxative in fruit.

The best way to profit is to secure a monopoly. For plant pollination, this involves picking a species that you can count on and courting it until it goes to no other. Or, at least, making yourself a favorite.

Bee-pollinated flowers are designed specifically to attract bees: yellow, blue, and violet colors, as well as ultraviolet patterns that only bees can see. Bee flowers typically offer a landing platform, with petals that form a tube specifically configured to allow nectar access only to insects with specialized mouth parts.

Pollination is not the only benefit that bees bring to plants. The buzz of bees about flowers discourages caterpillars, who fear the sound may be a predatory wasp.

Beetle-pollinated flower plants tend to have an open structure, with short mouth parts that beetles can readily access. Some have specialized food structures, such as clusters of cells with a flower that a beetle can eat.

Fly and beetle-pollinated flowers are often strongly scented, smelling of other fare preferred by these insects, such as dung and rotting meat. Orchids are common cultivators of stink-loving pollinators. The blunt-leaved orchid depends exclusively on mosquitoes for pollination. This orchid attracts its clientele by smelling like human body odor.

One benefit of a select pollinator clientele, besides a reliable relationship, is a steady transfer of pollen between flowers of the same species. It’s no good to give up pollen that has little chance of landing on a potential mate. Insect pollinators such as bees and butterflies do engage in flower constancy: transferring pollen to other conspecific plants.

This pollination specialization can be taken to extremes. Some plants have exclusive pollinators: a single-species symbiosis. Fig plants are absolutely dependent upon pollination by minute fig wasps.

The purple-throated carib hummingbird is the sole pollinator of 2 Heliconia species. Each hummingbird sex feeds at the flower species that matches the size of its bill. Sexual dimorphism met coevolution between plant and animal. Heliconia speciated to meet the specific needs of just 1 gender of hummingbird. Meanwhile, female and male hummingbirds adapted to a certain flower.

Plants are judicious retail mavens in their pollination practices: giving up only enough to keep the pollinators coming back and restocking at a speed expected to keep up with demand.

Biotic pollination is a tremendous benefit to plants widely distributed in small populations. Without it, populations would likely perish. With animal-aided pollination, plants can succeed in places which would otherwise be perilous.

Old-school angiosperms that depend upon wind pollination, now only 10% of flowering plants, run some risk from an unreliable ally, but need not bother with sizzling signage, and so can conserve their resources by not resorting to consorting with animals.

As in all things, there are always tradeoffs. Plants have a long history of unsurpassed adaptability, coupled to chemical proficiency and genetic manipulation savvy, that has let them make intelligent choices well beyond the reach of other macrobial life. Microbes have comparable sophistications in these realms, but scale presents unique challenges.

Plants can precisely tune their gene expressions to get the desired flower shapes, colors, patterns, and scents, as well as producing specific secondary metabolites that serve specific needs. How it is that plants understand their target audience’s nervous and digestive systems is outside the realm of empirical investigation, as Nature is an exposition involving sources unobservable.

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In plants, somatic stem cells give rise to male and female germ cell lineages that only differentiate late in development. In flowering plants, this occurs after formation of the floral organs, in which separate meiosis give rise to haploid unicellular male and female gametophytes – the microspores and megaspores – which undergo distinct germline developmental programs to form the gametes. The male germline is segregated in the gametophyte by asymmetric division of the microspore to form the generative (germ) cell, which rapidly establishes a distinct developmental program. This male germ cell then completes a mitotic division and differentiates to form the 2 sperm cells required for double fertilization. In contrast, the female germline is only segregated after 3 rounds of nuclear division followed by cellularization of the embryo sac. ~ English botanist Michael Borg et al

Seed Production

To see things in the seed, that is genius. ~ Chinese philosopher Lao Tzu

Plant seeds are a biological system that harkens back to a microbe talent: dormancy. Seeds withstand harsh environmental conditions for extended periods by greatly reducing their metabolism.

Lack of water is common problem facing germination, so seeds are parsimonious: the water content of maturing seeds is less than 10%. To protect genetic material from dehydration, a seed’s chromatin compacts and the nuclei of seed cells contract when seeds start to mature.

In gymnosperms, a seed contains a fertilized embryo and tissue from the mother plant, which, in coniferous plants, forms a protective cone.

Angiosperm seed production took a major step forward by providing endosperm: food for a germinating embryo until its roots can be established. This is accomplished via a uniquely intricate process called double fertilization: a single pollen grain with twin sperm are delivered by a pollen tube to an ovule inside a pistil. While one sperm fuses with the egg to generate a zygote, the other merges with the central cell to produce the endosperm.

Double fertilization requires a perfect union between a single ovule and a single pollen tube. Other pollen tubes wait to see if the current candidate is successful. If not, they have their go at the ovule until the deed is done.

Immediately following the moment of gamete fusion – when the 2 sperm cells from a pollen tube unite with the 2 female gametes in the ovule – all other pollen tubes are repelled and redirected toward unfertilized ovules.

As soon as the fusion is successful, a mechanism is triggered that tells all the other pollen tubes to go away. ~ American cytologist Mark Johnson

No physical, electrical, or chemical change communicates consummation: fertilization status is conveyed energetically. It is an essential communiqué.

This mechanism prevents the delivery of more than 1 pair of sperm to an ovule, provides a means of salvaging fertilization in ovules that have received defective sperm, and ensures maximum reproductive success by distributing pollen tubes to all ovules. ~ American cytologist Kristin Beale

Right after fertilization, the zygote is mostly inactive. The endosperm undergoes a spurt of development then goes into stasis. Then the seed is covered with a coat from maternal tissue.

When a seed begins to sprout, the endosperm sacrifices itself by feeding the embryo; but sometimes it holds back. Though extremely rare, the embryo and endosperm can be fertilized by sperm from different plants (heterofertilization). An endosperm that does not share the same father as the embryo does not fork over as much food. It is less altruistic.

Heterofertilization demonstrates the importance of motherly attention in ensuring proper seed development. As paternal interference can be counterproductive, the maternal genome takes control of the critical epigenetic regulators for seed development. This is not the only way that vegetative mothers look after their offspring.

Maternal Care

Mother plants may care for their seeds by guiding them about the best time to sprout. Rockcress mothers give their seeds their own memories of recent seasonal temperatures.

Rockcress in warmer climates produce seeds that sprout more quickly than those in cooler climes, even if the warmth occurred only weeks before mothers make the seeds.

To control their seeds, rockcress regulate the tannin in their fruit. Tannin determines how strong seed casings are. A higher tannin level makes a shell harder to break through, delaying germination.

The mother defines how hard the seed coat is to break free from, and in this way it’s controlling what the seed does. ~ English geneticist Steven Penfield

Establishment

Plants face considerable obstacles establishing themselves on new land. And it is tough for a plant to germinate in a community already heavily populated with other plants. Resident vegetation does not welcome strangers.

Disturbed soil facilitates germination. A seed lies fallow on hard ground. Seeds with greater mass do better under competitive conditions. Fast growth helps, at least at first.

Once a population has taken root, soil disturbance and standing biomass have little effect on the number of individuals in a species community. The main factor early on in increasing population size is propagule pressure: the number of seeds sown.

As the years pass, fecundity matter less. A plant community’s prospects increasingly depend upon surviving herbivore onslaught.

Over time, perennial plants fare better than nonperennial ones. Native species naturally have an edge over aliens; an advantage that increases as years go by.

Plant Adaptabilities

Plants are acutely aware of their environment. ~ American botanist Daniel Chamovitz

Plants have persisted, relatively constant, across extinction events that radically altered the mix of animal life. When climactic events affected plant populations, they were always quick to make a comeback.

The reason is that plants have basic needs: some sunlight, water, carbon dioxide, nitrogen, magnesium, phosphorous, potassium, and a few trace elements. This list is short compared to animals. These necessities are universal among all plant groups, and unchanged through time.

Plants are relatively insensitive to the population size dynamics that can afflict animals. A few individual vascular plants near each other fare well compared to a diminished deme of tetrapods.

Other techniques that have conferred a timeless robustness to plants include water management, asexual reproduction, polyploidy, hybridity, and dormancy.

Water Management

For a plant, water’s evaporative quality is a blessing and a curse. Atmospheric evaporation rates swing widely with weather and season.

Most primitive (non-vascular) plants are poikilohydric: tolerant of large fluctuations in hydration. Atmospheric CO2 concentration was higher when these plants evolved, as was photosynthesis efficiency as an outcome of more CO2. Hydration was relatively less important.

A few higher plant species are poikilohydric. Leaf photosynthetic efficiency declines with tissue water loss, recovering after some duration (days, months, even years) after tissues have rehydrated. Animals are much more tightly tied to time than plants.

The most important evolutionary innovation enabling plants to limit water loss was the cuticle: a land plant’s exoskeleton. Most land plant stems and leaves are covered with a waxy, waterproof film of chitinous material.

To enable controlled gas exchange between leaf and air, particularly a port of entry for CO2 during photosynthesis, plant epidermis has stomata (controllable pores). Stomata also serve for temperature control via transpiration.

Several plants slather their leaves with wax to control water intake. The wax repels water.

Lotus and some other plants took this hydrophobicity a step further, in shaping the wax coating in a way that effectively makes it self-cleaning. The wax is arranged with cones 5,000th of a millimeter high, with fractal patterns on the cones at an even smaller scale.

When water lands on these waxed leaves, they cannot stick at all. Instead, the water forms spherical drops that roll across the leaf, picking up dirt along the way, until the drops fall off the leaf. This ultrahydrophobicity is called the lotus effect.

Water management factors heavily in handling stress, both regarding water availability and temperature extremes. We look at those plant management strategies a little later.

Reproduction

A vascular plant typically grows from a seed, the product of sexual reproduction. Earlier-evolved species, such as fern, sexually reproduce via spores.

Spermatophytes are plants that produce seeds. There are 2 sorts of seed-bearing plants: gymnosperms and angiosperms. Pines and other conifers are gymnosperms: Greek for “naked seeds.” The seeds of flowering plants are a step up, as they contain the nutrition that the seed embryo needs to get its start in life: endosperm.

Asexual reproduction is extremely common in plants. This vegetative reproduction has been a critical adaptation for the persistence of vascular plants through time. Many plants reproduce sexually or asexually depending upon conditions.

Plants that reproduce asexually produce clones: offspring that are genetically selfsame to the parent. Offspring clones may be physically linked or not to the parent.

Linked clones are outgrowths of the parent. The linkage in these clones is broken at some point, separating the parent from its offshoot offspring.

Non-linked clones are plant parts that are dispersed. Plant bulbs are a non-linked clonal form.

Different species use various techniques for vegetative reproduction, including budding, suckering, stolons, and rhizomes. In budding, a stem grows into a new plant. In suckering, a plant regenerates or reproduces by new shoots from an existing root system.

Other vegetative reproduction involves tissue specialization. Stems on the ground, termed stolons, shoot out from some plants to create a new individual (ramet). Strawberries reproduce via stolons.

In some species, a stolon ends with the growth of a tuber. A tuber is a swollen stolon that forms a new plant.

Potatoes are the most famous tuber, formed from a stolon. Besides tubers, potato plants can also reproduce vegetatively by budding. Potato plants also reproduce sexually via flowering.

A rhizome is an underground creeping rootstalk that modifies to act as an organ of vegetative reproduction.

Stolons and rhizomes differ somewhat. A rhizome comes from the main stem of a plant. A stolon sprouts from a secondary stem.

A stolon tends to have longer internodes than a rhizome. An internode is the growth between the nodes which form new plants.

A rhizome sends out roots from the bottom of a node, and upward-growing shoots from the top. A stolon simply generates new shoots at the end, such as in a strawberry plant.

Asexual reproduction yields at least 2 survival edges. 1st, sterile plants, such as certain hybrids, may become widespread and persist. 2nd, cloning copes with stress. Owing to cloning, many plants can come back from trauma, such as burning, cropping, or irradiation. Cloning enables plants to persist in the face of adverse environmental conditions, even disease in some instances.

Historically, in periods when global conditions were conducive to plant growth, such as the Carboniferous (354–290 mya), mid-Mesozoic (220–200 mya), and early Paleocene (65–40 mya), aclonal plants increased. In contrast, when conditions became more arid, as in the late Permian to early Triassic (256–242 mya), and from the Miocene (23 – 5.3 mya) to, and through, the Quaternary (24–0 mya), clonal species multiplied.

Monocots (monocotyledons) have a solitary cotyledon: a single embryonic leaf in their seeds. Dicots (dicotyledons) have 2 leaves per seed.

Clonal reproduction dominates in angiosperm monocot families. The various grasses are exemplary.

Dicots tend to be dominated by aclonal plants. Dicots include the most common garden plants, shrubs and trees, and broad-leafed flowering plants such as magnolias, roses, and geraniums.

With global warming and increasing aridity, monocots may become the dominant global flora. Altering the mix of planetary plants will profoundly affect terrestrial animal life.

Polyploidy

Most eukaryotic species are diploid: 2 sets of chromosomes, 1 from each parent. Humans are diploid, though some tissues, including the heart muscle and liver, may be polyploid: more than 2 sets of cellular chromosomes.

When polyploidy occurs in animals, it is often as a mistake in mitosis (cell division). But there are a few polyploid animals. Flatworms, leeches, and brine shrimp are commonly polyploid, as are some salamander and lizard species.

Polyploid animals are typically sterile, which can be overcome by parthenogenesis: asexual reproduction where an unfertilized egg cell nonetheless develops into an embryo.

Polyploidy is pervasive in plants. Somewhere between 30% to 80% of living plant species are polyploid.

The adaptive radiation of angiosperms 100 mya coincided with genome duplications shared by many species. This conferred advantages.

New species can evolve via polyploidy within the home range of the parent species.
In polyploid plants, potentially harmful recessive mutations are more readily masked, rendering plants more resilient, and resulting in low phenotypic variability in populations.
With a vaster genome, polyploid plants have a greater store of built-in genetic variability. With this innate knowledge base they can more readily adapt and cope with environmental extremes. Compared to their non-polyploid parents, polyploid offspring are geographically more widely distributed, and better able to live at habitat margins.

Concomitant with polyploidy is the ability of plants to undergo mosaic evolution: different organs on the same plant can divergently develop, independent of each other. Under environmental stress, a plant can protect its whole genetic package by selectively altering 1 or a few organs while leaving others unchanged. Such modularity affords a plant flexible adaptability.

Hybridity

Like microbes, plants have the ready ability to select specific genetic material from others to hasten their own evolution: to create a hybrid, bridging 2 different varieties or species into something new. Hybridization serves to both perpetuate populations and evolve new species specifically adapted to current environmental demands.

Eucalyptus, oak, bearberry, and mountain lilacs and lilies are exemplary. On the west coast of California, distinct species of mountain lilac developed their niches for specific climactic conditions via hybridization.

The elegant sego lily prefers sandy aridity at mid-elevations in the Rocky Mountains, in a somewhat open canopy of ponderosa pine forests. The mariposa lily, with lovely cream-colored petals, situates in moister sites at higher elevations, under the more closed canopies of Douglas-fir.

Interspecific hybrids between the 2 lilies are abundant on ski slopes in western Montana, where Douglas-fir canopies have been opened and kept clear of trees and tall shrubs. The lilies adapted by hybridization to a human-created hybrid habitat.

Unlike animals, quite divergent plant species, even of different genera, have recombined. Such cross-genera hybridization is relatively rare, since different genera are genetically and phenotypically more divergent than species in the same genus. The shrubs bitterbrush and cliff-rose combine to form the hybrid desert bitterbrush, which is distinct.

Dormancy

Seed dormancy, by controlling the timing of germination, can strongly affect plant survival. ~ American evolutionary ecologist Charles Willis et al

When favorable conditions flag, leaves or whole-branch portions of plants can be shed. Under extreme stress, plants can die back to the ground and perennate underground as stems or rhizomes.

Plant seeds are even better designed to preserve through dire times. Coherent adaptation birthed bright seeds at least 360 million years ago. Even the earliest seeds had the sense to wait for fortuitous circumstance until starting on life’s adventure.

Under decent conditions, many plant seeds remain viable for a century or more. 2/3rds of ancient sacred lotus seeds recovered from a lakebed in Liaoning province, China, 330–1,200 years old, germinated and grew into mature plants.

 Looted Fruit

An arctic ground squirrel stashed in its burrow a fruit from a narrow-leafed campion, a small flowering plant. 31,800 years later, the burrow was looted in the name of science.

The fruit seeds failed to germinate. But living plants were propagated by cloning material from the fruit’s placenta, which produces seeds.

The pillaging Russian researchers belatedly lamented the loss of the savvy squirrel, who smartly packed the fruit in a larder strategically positioned next to permafrost, assuring a year-round chill of preservation. In the intervening duration, windblown soil had buried the burrow 38 meters, permanently freezing it at –7 °C.

Seeds from the regenerated ancient plants germinated with 100% success, compared to 90% to modern plant campion seeds. The only apparent difference between the modern and earlier plants was that the earlier-evolved flowers had narrower, more splayed petals.

Plant Intelligence

Plant behaviour is active, purposeful, and intentional. The plant gathers information about its surroundings, combines this with internal information about its internal state and makes decisions that reconcile its well-being with its environment. ~ Anthony Trewavas

Intelligence is demonstrated by behaviors. Plants are so different from animals that is it commonly thought that plants do not behave at all.

Animals skitter about, bodies and limbs in motion, their communications in sounds, gestures, postures, and expressions. Chemical processes within proceed largely outside animal awareness and control. Decisions regularly take place unconsciously, only coming to awareness in their fruition.

In contrast, phenotypic plasticity and chemical production predominate plant behavior. Plant gestures and expressions are unrecognizable to us.

Plants use microfluidics and optics to move, change color, and pump water. ~ Greek American electrical engineer Demetri Psaltis

Visible plant actions largely comprise growing and discarding parts. Production and allocation of chemical resources is crucial plant behavior.

Plants intake and integrate information from among their various parts, combine it with remembered and genomically available knowledge, and intelligently make decisions. Because plant behavior is largely chemical and phenotypic, the number of choices that a plant at any moment has dwarfs any analogue of animal behavioral options.

Plants live a life of conscious chemistry. Their thoughts and behaviors are exercises of molecular awareness. The contrast to animals is incomparable.

The genomes of DNA-containing cell organelles (mitochondria, chloroplasts) can be laterally transmitted between organisms, a process known as organelle capture. Organelle capture occurs in plants. ~ Belgian biochemist Sandra Stegemann et al

As molecular mavens, plants comprehend the meaning of the informational codes behind genetics. They examine all the DNA that comes to them – whether bacterial, fungal, animal, or from another plant – to determine whether it may have value. This partly explains their ability to establish and regulate relations with other life forms.

Plants control their own genetic destiny: manipulating their genome in a vast variety of ways to achieve goals.

Part of identity is what you aren’t. Especially for plants because they are so changeable and susceptible to environmental conditions, the part of the genome that is not needed, or that might be providing exactly the wrong information, needs to be shut off reliably in each condition. This information is then passed on to daughter cells. ~ American microbiologist Doris Wagner

Deciding priorities and energy allocations is so complex that no plant behavior is autonomic. Unlike animals, there is no plant unconscious.

One aspect of existence that is the same for both plants and animals is memory. Plants remember their ecological interactions and derive meaning from them. Plants have long-term memory.

Animals process memories when they sleep. Plants rest during the night, but it is not known whether this helps them incorporate memories.

As with animals, the lessons that traumas may teach need to be learned, but the emotional impact of traumas must be set aside if a plant is to recover and lead a healthy life.

Stress memories may be maladaptive, hindering recovery and affecting development and potential yield. In some circumstances, it may be advantageous for plants to learn to forget. ~ Australian botanist Peter Crisp et al

Animal emotions play critical roles in memory retention, judgment, and motivation. Evidence of plant emotions is anecdotal, but the evolutionary advantage of emotions is such that it is hard to imagine that plants lack emotive feelings. Plants demonstrable will to live suggests there being an emotional context to their behaviors.

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Plants plan. Decisions about growth or defense are processes of potentiality, aimed at meeting anticipated needs.

Plants anticipate attacks from insects in much the same way that they anticipate the sunrise. ~ American biologist Michael Covington

Assessing the far-red radiation coming off the leaves of competitors, plants can predict potential loss of light in the foreseeable future. One intentional response is shade avoidance – a goal-oriented behavior.

There is an extensive spread of prerain green-up over Africa. ~ Nigerian terrestrial ecologist Tracy Adole et al

A swath of Sub-Saharan Africa has a rainy season with an attendant bloom of vegetation. Plants there anticipate rain coming and green up before the rain arrives. They also know when the rainy season ends and lose their lushness just after the rain stops, thereby conserving their resources.

Associative learning is an essential plant behaviour. ~ Australian biologist Monica Gagliano et al

Experience and calculation of relative gain determine decisions in plants just as they do in animals. Plants constantly assess the probabilities of favorable outcomes given an ample array of possibilities for root and shoot growth vis-à-vis defensive measures. Plants take risks when they feel they need to.

Competition plays a fundamental role in plant ecology. Plants evolved both the ability to detect the presence of neighbours and to plastically adjust their phenotypes in response. Plants can respond to light competition in 3 strategies, comprising vertical growth, which promotes competitive dominance; shade tolerance, which maximises performance under shade; or lateral growth, which offers avoidance of competition. Plants choose according to outcome. Plants adopt optimal scenarios. ~ German botanist Michal Gruntman et al

 Pesky Pollinator

The coyote tobacco prefers to rely upon the hawkmoth for pollination, as the moth visits many plants in its wide-ranging forays. The problem is that the hawkmoth is both pollinator and pest: it lays its eggs on the plant, and its larvae love eating tobacco leaves. Those plants which best reward hawkmoths with nectar are most likely to have eggs laid on them; a cruel irony indeed.

The coyote tobacco courts the nocturnal hawkmoth by opening its flowers at sunset and wafting an alluring scent. This benevolence shuts down when hawkmoth larvae (tobacco hornworms) make tobacco leaves their meal ticket. The plant produces specific pesticides which decrease the caterpillars’ digestive ability.

As a final gesture of disgust, the plant stops flowering in the evening and opens for business at dawn, thereby attracting hummingbirds. Hummingbirds may not be as prolific a pollinator as hawkmoths, but at least they don’t eat you alive. Once a coyote tobacco is no longer losing leaves to hornworms the plant goes back to preferring hawkmoths as their pollinating pals.

 Killer Quorum Mimic

To have any chance of success, bacteria attack plants en masse. Little pathogens determine that they have the numbers on their side via chemical quorum-sensing communications.

Plants understand quite well how bacteria operate. To thwart a mass assault, plants concoct and release chemicals that mimic those used by bacteria to signal each other that the time to attack has come.

A plant fires off its molecular mimic before bacteria have sufficient numbers to tackle their target. The microbes invade before they have enough troops, whereupon the plant picks them off.

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Typically, only 1–3% of light absorbed by a flowering plant is converted to chemical energy, though it may run as high as 8%, as in sugar cane. Much of the solar energy instead goes to heating and pumping water. The reason is that plants practice a form of endothermy: keeping leaves at 21.4 °C, which is optimal for photosynthesis. Trees ranging from Alaska to Mexico keep their leaves at the ideal temperature.

Plants have sophisticated strategies to secure resources. Nitrogen is exemplary of supplies that are heterogeneously distributed in the environment. If a root senses a local shortage pending, it consults with a shoot that it services, which may suggest lateral root growth toward regions that promise better nitrate uptake.

Plants integrate local and global nutrient cues to spend resources efficiently. ~ Dutch botanists Ton Bisseling & Ben Scheres

While some determinations are made that affect a whole plant, many are local, especially in a relatively mature plant with more resources at its disposal. Plant intelligence is therefore a fluid mix of holistic and decentralized decision-making.

An experienced root tip, having encountered a situation before, knows the best way to proceed. A poplar leaf, having been scarred by an insect thug, kicks into defensive emission faster than a naïve leaf.

 Barberry Battles Barbarism

The barberry is a European shrub that produces berries with 1 or 2 seeds; typically 2. The plant has the ability to halt the development of its berry seeds.

The parasitic fruit fly Rhagoletis meigenii punctures berries to lay its eggs inside. The barberry is aware of a berry being infested.

If one of the 2 seeds in a berry is infested, a barberry aborts the infested seed to save the uninfested one. But if the berry only has a single seed, the plant gambles that the larva may die, which is a possibility. After all, losing a 1-seed berry is an utter waste of fruit.

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Photosynthesis during the day lets a plant grow and pack away enough energy to last the night. When the Sun goes down, based upon its starch storage, a plant precisely calculates, on a leaf-by-leaf basis, how much energy it can use until dawn.

Optimizing efficiency, each leaf allocates its resources over time via arithmetic division. Using its internal clock, a plant monitors its accuracy by repeated calculation during the night, adjusting as necessary.

If the starch store is used up too quickly, a plant starves and stop growing. Conversely, too-slow consumption is a waste. Plants get it right, but caution is applied. Leaves leave a 5% contingency reserve.

The computational capabilities of plants are enormous. Plants uses their genome as an active database, precisely manipulating their own epigenome for remembrance.

Flowering plants remember induced states for long durations via various mechanisms, including altering the chromatin at certain genetic loci, through self-reinforcing protein modifications, and cellular memory via stable developmental or metabolic states. These are just evidentiary artifacts to the vibrant energy system that lies behind all living matter.

Growth represents resource investment. The best return-on-investment is a choice that offers relative stability.

Plants consider context. Trees manage to grow in well-spaced patterns, as a walk through the woods readily shows. Various feedback mechanisms prevent overcrowding.

Plants learn and remember which specific growth patterns are most productive. Some plants with a natural propensity toward spindly growth get bushier when pruned. Having sensed that they are somehow spatially confined, plants adjust based upon experience.

Physically, memory meets action potential by electrical impulses, analogous to animal nervous systems. Ionic signals propagate through plants cells, provoking chemical changes that often incite physiological and morphological transformations.

There are thousands of interrelated states throughout a plant. Changes in states are coordinated.

Different plant stems have varying success with their leaves harvesting light. While bright light typically encourages growth in that direction, shoot growth is halted when heading into the shade.

A plant makes growth decisions based upon all the information available to it. To optimize nutrient harvest, data from individual roots are collated to determine an overall growth pattern for further root foraging.

Communication

Molecular transport of plant hormones and other signals in cell-to-cell communication is slow. Yet information is propagated quickly over large distances such as a tree height or width. Moreover, billions of cells building up a tree must quickly and efficiently communicate with one another. This is a necessity for maintaining this large organism’s integrity throughout its development. ~ Polish botanist Katarzyna Sokołowska

A plant is itself a communication network. Its internal signaling system is analogous to the human brain, though more sophisticated.

Plants may employ both chemical and electrical signaling to spread news. The major neurotransmitters of animal nervous systems are also in plants, including serotonin, acetylcholine, GABA, and glutamate.

One way electrical messages travel is through the vasculature that conducts water and organic compounds. Leaves may also generate electrical waves via continuous relay of cell membrane depolarizations.

Plant electrical signals travel up to 9 cm per minute. In contrast, the mammalian nervous system can rely electrical signals at 100 meters per second. But then, such speed is essential for animals that rely upon locomotion to stay alive and cannot survive the extent of damage that plants can.

Plants also have an equivalently rapid extracellular communication system, using waves of reactive oxygen species.

Signaling may be highly selective. An insect larva may not raise an alarm by walking on a leaf. But once it starts to munch the greenery, neighboring leaves are notified.

Leaves on a plant listen to the experiences of others. Even the rumor of distress can create a state of preparedness in a leaf.

 The Headaches of Willows

The ancient Greeks, Middle Easterners, and Native Americans all eased aches and reduced fevers by ingesting the chemicals in willow tree bark. The active ingredient is salicylic acid, which is a chemical precursor to aspirin.

Plants do not use salicylic acid to ease their pain in the same way humans do. Instead, they release it with pinpoint accuracy at an infection site, signaling veins throughout the plant of a bacterial or viral attack. Salicylic acid is a localized self-sacrifice gambit to thwart an invasion.

The plant responds with several steps to kill the invader, or at least stop the spread of the infection. For one, dead cells are put up around the infection site as a barrier, to halt movement to other plant parts. White spots on leaves are an indication of where plant cells have killed themselves to prevent bacteria from spreading.

Lines of Communication

Plants form communication lines among themselves and their neighbors. This provides a direct early warning system for threats in the neighborhood.

Most land plants are connected indirectly through mycelial fungi. The relationship between the fungi and plant is mutualistic.

Besides nutritional benefits, plants employ the fungi as communication conduits. For plants without runners, mycorrhizae make a fine chat line.

Such communication improves a plant’s fitness. Plants are better able to prepare themselves for attack and more successfully avert damage.

When attacked, plants emit volatile airborne compounds that warn of their situation and solicit help from the natural enemies of their assailant.

Neighboring plants eavesdrop on each other and respond by priming their own defenses. They may do so quite selectively.

The plumbing of poplars is such that leaves do not have a direct connection to their neighbors. But volatiles released by a leaf under attack waft to neighbors, divulging data about the nature of the attacker.

Should an attacker move to graze nearby, the new prey revs its defense faster. The plant itself benefits by its own emissions but so do neighboring plants.

 Others on the Party Line

A plant communication network may be co-opted. Plant viruses use runners as highways to rapidly spread through connected plants.

Even a single plant acts as a transmission line for herbivorous insects. An underground pest broadcasts chemical signals up through the leaves of a plant, telling insects aboveground that the plant is already occupied.

Leaf-eaters prefer plants that are not infested by subterranean insects. Avoiding competition keeps a healthier habitat by not overtaxing plants and thereby unnecessarily damaging the community at large.

Parasitic wasps lay their eggs in caterpillars that live on stems and leaves. These wasps benefit from the signals of underground insects, as it helps reveal where they might find a good host for their eggs.

Chemical Calling Cards

Some plants release chemicals that resemble insect pheromones: volatile chemicals employed in communication between social insects. When set upon by aphids, wild potatoes cry out with a compound that acts as an alarm pheromone for aphids. The aphids flee the potato plants.

Corn seedlings ask for assistance when attacked by armyworms, releasing a pheromone that attracts female parasitic wasps which feed on armyworms.

A lima bean plant attacked by beetles has a 2-pronged response. 1st, leaves under attack spread the alarm to undamaged leaves to prepare for assault. Neighboring plants “leavesdrop” and steady themselves.

The lima bean’s leaf alarm is methyl jasmonate, a defense hormone in airborne form. In contrast, a plant under attack by bacteria exudes methyl salicylate, a gaseous equivalent to salicylic acid.

2nd, the lima bean plant has its flowers, which the beetles don’t bother, produce a nectar alluring to beetle-eating arthropods. Many insect-eating arthropods, as well as pollinators, coevolved with plants, and came to share chemical communiqués of mutual benefit.

Predators of plant pests can be picky, selectively flocking to the aromatic news of a menu option, while ignoring scents that signal species they don’t fancy.

A little wasp that injects its eggs into young caterpillars reacts to attacks when a plant’s panic aroma is of tender young caterpillars. But a wasp turns a deaf ear to a plant screaming from attack by geezer caterpillars.

Cheater Compliance

Plants have a variety of contractual relations. If an equitable quid pro quo is not met, a plant is likely to enforce compliance or cancel the contract altogether.

 Legumes & Rhizobia

Legume symbiosis with rhizobia is the largest source of natural, non-synthetic, nitrogen fertilizer in agriculture. ~ American plant pathologist Nevin Young

Legumes employ bacteria, collectively called rhizobia, to fix nitrogen for them. In return, the bacteria are given comfortable accommodations in root nodules. Neither rhizobia nor their plant hosts are dependent upon one another to survive.

The start of their symbiosis, known as nodulation, involves intricate negotiations between the two: chemical communiqués, and abundant alterations in gene expression and development patterns in both organisms as a nodule forms and the bacteria are ensconced in a comfy new home. Nodulation is a complex, completely cooperative process of accommodation by both parties.

The ability of bacteria to form this intimate association with plant cells owes to the plant not activating any defenses against them. The bacteria are particularly recognized by the plant. Uncooperative rhizobia are unable to infect host plants, as they trigger host defenses.

A nodule is a workshop where nitrogen fixation can occur efficiently. An optimal oxygen concentration is maintained within. O2 regulation is itself a complicated process involving coordinated collaboration between host plant and bacterial guest.

Once the relationship is figuratively and literally sealed, a legume trades sugars and proteins in return for usable nitrogen from the rhizobia. The ammonia burped by the bacteria is employed by the plant to synthesize amino acids.

Nitrogen fixation in this cooperative relationship is unparalleled in its communication complexity and coordination between organisms and is all the more remarkable for transpiring between very different life forms.

The legume/rhizobia nitrogen fix was facilitated 58 MYA, when a duplicate set of genes of the whole plant genome were created. This allowed conservation with the original set, while the duplicate developed the new functionality needed.

A single legume plant is typically host to several different bacterial lineages. Some are more industrious than others. Strains that fix little or no nitrogen are common in some soils.

Plants carefully monitor nitrogen production by their rhizobia. Some plants practice a strict quid pro quo: providing carbohydrates in accordance with the nitrogen supplied by the bacteria. If a plant becomes dissatisfied with a rhizobium slacker, the host sanctions the bad bacteria by reducing its oxygen supply.

 Yuccas & Moths

Yucca plants are pollinated exclusively by yucca moths; a mutualist relationship that began at least 40 million years ago. In return for their pollination service, female yucca moths lay their eggs in the flower.

The life of an adult yucca moth is so short that it does not need to feed. These moths lack the long tongue characteristic of most moths and butterflies. Instead, the yucca moth has tentacles around its mouth that let it serve as a pollinator.

A female yucca moth follows a specific pollination procedure. First, she visits the anthers of flowers, scraping pollen from several of them and shaping it into a lump. She then leaves to land on the flower of another plant, assuring cross-pollination. When she arrives, she inspects the flowers, looking for one at the right stage, and for one that does not already have eggs in them.

Female yucca moths signal floral egg status to each other. A yucca moth can detect the telltale smell of other females with her antennae. Females typically leave previously visited flowers without ovipositing.

Finding a proper flower, a female lays a single egg, or a few at most. Afterwards, she goes to the stigma, where she carefully removes some pollen from under her chin and deposits it. Now the flower can produce a fruit and enough seeds to feed her larvae as well as ensure plant reproduction.

Upon hatching, a yucca larva has a ready supply of food in the form of Yucca seeds and other plant material. While the plant loses some seeds, it gets the better of the bargain. This is particularly true because the Yucca plant won’t tolerate being over-exploited. The Yucca counts the eggs within its flowers. If there are more than a few eggs in any particular ovary, the plant aborts the flower, killing the moth offspring inside.

What the plant discovers is seldom satisfactory. 70% or more of the pollinated flowers have an unacceptable number of eggs and are aborted. A shot of ethylene ensures their demise.

While relatively few yucca eggs mature to adulthood, their chances are much better than being deposited on altogether uncaring plants. The mutualism, however occasionally antagonistic, is obligate: each depends on the other for survival.

The high mortality rate of yucca eggs owes in part to the fact that there are cheater species of yucca moths who provide no pollination service. These moths are only distantly related to those that play by the rules.

The cheaters have been around for a long time. As a few manage to succeed, statistical odds have kept them in business.

Senses

Plants can distinguish between one stimulus and another. Plants clearly have the means to tell different types of signal apart. ~ Brian Ford

Everything alive relies upon sensory input to decide how to behave. All that senses are, after all, are receptors for perceptual cognition.

The mind creates a mosaic from a multitude of pinpoint sensory observations. Plants are no different from animals in this regard. That said, floral level of awareness is superior than that of fauna.

Plants have to be better than us at sensing the environment because they don’t have the luxury of getting up and leaving. ~ American botanist Simon Gilroy

Sight

Plants search for food as if they had eyes. ~ German chemist Justus von Liebig

Plants can tell the level of light, its specific direction(s), and its wavelength composition. From this, plants know the time of day, and the time of year.

Photosynthesis produces sugar. One way that plants know the time of day is by the rate of sugar production, which they continuously tally.

Angiosperms use the sense of season to time their flowering. To optimize the prospect for propagation, plants synchronize flowering with pollinator life cycles.

All photosynthetic plants grow toward light, thereby enhancing the photonic fuel for photosynthesis. This is phototropism. Plants adjust growth direction by elongating the cells of the stem on the side farthest from the light source.

A plant knows whether another plant has grown over it. Its typical response is to grow faster, to regain access to better light.

Besides being an energy source, light serves as an important signal to rationally regulate growth. A wide variety of processes throughout a plant are mediated by light-signaling molecules. Even roots are affected.

Light is first detected by photoreceptors in the shoots of a plant. Roots have low-wavelength photonic receptors that are activated by light transmitted from the shoots via vascular bundles. Thus, the entire plant is exposed to light cycles and can plan accordingly.

In essence, plants see, though their sight is naturally oriented toward what a plant needs to know. But then, animal vision is the same in its orientation: limited to spectral bands with needed information, while not able to sense frequencies that are superfluous to survival.

Facing the Dawn

From dawn to dusk, many flowering plants track the Sun across the sky. Plants with leaf mosaics, such as trees and vines, carefully contrive to arrange their leaves so that each gets its fair share of sunlight.

The mallow plant keeps its leaves flush to the Sun all day long, soaking up the light. After the Sun sets, a mallow spreads it leaves conventionally: facing upwards. But as dawn comes due, a mallow turns its leaves to the east in anticipation of sunrise. Such behavior is typical of Sun followers.

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Many flowering plants are quite sensitive to light. While plants generally like light, there can be too much of a good thing.

The scarlet pimpernel opens its flowers at dawn, closing them after lunch time. In contrast, the evening primrose keeps its flowers closed during daylight, instead opening as dusk draws on.

Several plants vary their sunlight exposure based upon their ability to take the heat. The sirato orients its leaves to fully face the Sun when moisture is abundant. During drought, a sirato holds it leaves edge-on to the light, to minimize evaporation.

 Compass Plant

The compass plant, which grows on the North American prairies, takes a simple approach to getting just enough Sun. As flat new leaves grow, they align themselves on a north-south axis. All the leaves of the compass plant lie parallel to each other.

The Sun rises shining directly on one side of a leaf. At noon, light hits the leaves edge-on. As the day wears into the afternoon, the Sun moves around to shine on the other side of the leaf.

Compass plant leaves do not move. Their careful orientation means that they sunbathe in the morning and afternoon, while avoiding the scorching noonday Sun.

Touch

Touch is the sense of direct environmental contact. Plants are very much in touch with their habitat.

Roots can distinguish between wet and dry soil. They pattern their growth based upon the minute difference in moisture on one side of a root filament versus the other.

Plants have palpation perception, whether by the breeze or more substantial contact. The plant sense of touch extends to recognizing the saliva of herbivores that prey upon them. Touch is at its most developed in climbing plants, which possess an extraordinary tactile sense.

Unlike animals, plants are unable to run away. Instead, plants developed intricate systems to sense their environment and respond appropriately. Reactions can be triggered by rain drops falling, the wind blowing, an insect moving across a leaf, or even by clouds casting a shadow over a plant. Plants are very sensitive, and can redirect gene expression, defense, and their metabolism because of it. ~ Australian botanist Olivier Van Aken

Touch is ultimately an electrical sense. Established ion channels are energized by movement of fluid within and among cells.

The Venus flytrap literally has hair-trigger response to tactile stimulation. Charles Darwin showed that a Venus flytrap can be anaesthetized, just like animals. A flytrap gets back in action when the effect wears off. Ether, chloroform, or morphine may render a flytrap senseless.

Vines feel when they have latched onto something and initiate rapid growth when attached to reliable support. A bur cucumber can feel a string weighing 0.00025 mg, whereupon it sets itself to wrapping and growing. In contrast, it takes 8 times as much pressure for a human finger to sense a string.

Trees growing on a mountain ridge, exposed to high winds, adapt by limiting branch growth and growing short, thick trunks. In contrast, the same species of tree in an idyllic valley will be tall, thin, and have fulsome branches.

Heliconia tortuosa is a tropical plant that allows only 2 species of hummingbirds to pollinate its flowers. The plant knows who is guzzling its nectar by the shape of the bird bill put into its flower.

Plants generally don’t like to be touched. Simply touching or shaking a plant can lead to growth arrest.

A researcher studied cocklebur, a North American weed with small burrs that readily cling to passersby. This was done by measuring leaf length. The researcher found that the leaves measured never reached normal length. Instead, they turned yellow and died, just from being touched a few seconds each day.

Many plants are hardier about being touched than the cocklebur. Typically, those that are often touched keep their defenses up by producing defensive metabolites.

 Touch-Me-Not

Touch-me-not is hypersensitive to tactile sensation or rapid temperature change. It has bipinnate compound leaves, with 10–26 leaflet pairs per pinna.

When a leaf is touched, the leaflets rapidly fold inward and droop. If unprovoked further, a leaf regains its full posture after a few minutes.

Touch-me-not leaves have pulvinus cells at each leaflet base that facilitate the quick movement; a seismonastic response. The pulvinus act as hydraulic pumps.

When a leaflet is open, its pulvinus cells are full of water. A high concentration of potassium ions provokes water to flow into the cells from outside, in an attempt to achieve electrical neutrality by diluting the potassium.

Conversely, reacting to electrical action potential, potassium leaves the cells and takes the water with it when its channels are opened. The leaflet goes limp.

Once the signal has passed, the pulvinus pumps potassium into the cells again, along with water influx. The ions that regulate the pulvinus potassium channel are of calcium, which is the same element used for neural and glial communication in animals.

Calcium is the chemical of choice because of its high reactivity. It is energetically easy to incite calcium ionization.

Gravity

Animals grapple with gravity. To grow vertically, plants must accurately gauge gravity. They do so with aplomb.

The sensor of gravity in plants consists of tiny starch-rich grains called statoliths that sediment and form miniature granular piles at the bottom of the gravisensing cells. Despite their granular nature, statoliths move and respond to the weakest angle, as a liquid clinometer would do. This liquid-like behavior comes from cell activity, which agitates statoliths with an apparent temperature one order of magnitude larger than actual temperature. Active fluctuations of statoliths explains the remarkable sensitivity of plants to inclination. ~ French biophysicist Yoël Forterre et al

Smell

As consummate chemists, it is unsurprising that plants have a sense of smell. Whereas animals sniff odors within a second, plants require longer exposure to accumulate a scent. As with most things, plants are not in the same rush that animals are.

Smell is necessary to know the neighborhood, as so much of what goes on produces chemical signatures. A plant that smells a sick neighbor will put its guard up.

Male fruit flies emerge in late spring. They perch on the upper leaves of the tall goldenrod, and waft pheromones that attract female flies.

When these goldenrods get a whiff of male fruit flies, they prepare chemical defenses that make them less appealing to the females that can damage the plants by depositing eggs on them. These toxins deter egg-laying. The goldenrod’s defensive posture also makes it less attractive to other insects that might feed on it.

Floral scents are part of a complex trade-off with other scents that likewise attract beneficial insects. ~ Swiss botanist Florian Schiestl

Flowering plants can adjust their scent bouquet at any time to address immediate concerns. A plant can turn off its alluring floral scent and have its leaves request help from parasitic wasps if it is being attacked.

The Scent of Ripening

By altering the expression of one protein, ethylene produces cascading waves of gene activation that profoundly alters the biology of the plant. ~ American botanist Joseph Ecker

Plants time the ripening of their fruit by smell. Ripening fruit produces ethylene, which also plays a role as a hormone in the growth and aging processes of plant parts. Ethylene also helps a plant defend against pathogens.

Ethylene inspires uniform ripening, providing a hearty welcome to animals that come and distribute seeds in return for a meal. Providing only few fruit at a time would not be nearly as effective in attracting help, so plants host a feast.

In the early 20th century, Florida citrus growers ripened their product in sheds heated by kerosene. They were sure it was the warmth that prompted ripening. But, to their surprise, electric heaters had no such effect.

In 1924, USDA researcher Frank Denny discovered that kerosene smoke contains minute amounts of ethylene. He found that lemons can detect ethylene in as little as 1 part in 100 million.

Hearing

Plants respond to sound and they make their own sounds. The obvious purpose is communicating with others. ~ Monica Gagliano

Plants listen, attuned to specific sounds around them. Plant roots grow responsive to sounds in the soil. Plants distinguish between the wind and the crunch of caterpillars munching on foliage.

Leaves can detect a vibration less than 0.00025 mm. This allows them to detect and prepare for predators before they attack.

Nectar can be a significant energy investment, and thus keeping a constantly high level of sugar can be wasteful. ~ Israeli botanist Lilach Hadany

Plants hear a pollinator nearby and ramp the sweetness of the nectar in their flowers within a very few minutes. This economizes on unneeded sugar production and gives clients what they want.

The nectar response is frequency-specific: flowers respond to pollinator sounds, but not to other sounds. ~ Israeli botanist Marine Veits et al

When plants open their pores to capture carbon dioxide, they lose water. To replenish this moisture, roots suck groundwater and send it through xylem.

2-way valves – pit membranes – connect xylem capillaries. The drier the soil, the more tension builds in a xylem vessel, until an air bubble is pulled in through the membrane.

An embolism can be disastrous, as gas bubbles block water flow. Plants listen to their plumbing system and repair damage.

Sound is overwhelming, it’s everywhere. Surely life would have used it to its advantage in all forms. ~ Monica Gagliano

Electricity

Plants sense the electrical environment, and themselves use electricity as a communication conduit. For instance, Venus flytraps snap shut based upon electrical messages from triggered hairs.

Clouds carrying rain pronounce their arrival with electrical fanfare. As rain droplets grow they build up large negative charges. To take full advantage of rainfall, a complex cascade of enzymatic actions within plants take place in preparation. Dried leaves reactivate metabolism, getting ready to receive the water to come.

Flowers are a multifarious advertisement. Besides their good looks, scents, and feel, flowers emit electric fields which beckon.

In foraging afield, bees build a positive charge from flapping their wings. This charge helps pollen stick to them.

Alighting upon a flower affects its electric field for a time. From this a bee can sense how recently has flower has been serviced by another, and so determine whether the flower is worth the trouble of landing.

Sense of Self

Plants possess self-awareness: the ability to distinguish themselves from others. Flowering plants can tell their own pollen from that of another.

A plant knows which of its neighbors are related. Roots respond differently when encountering family versus strangers. Sagebrush plants, having sniffed wounded siblings, are better prepared for defense than from being exposed to warnings of genetically different plants.

Plants possess proprioception. A plant puts its roots down and its photosynthetic leaves toward the light, which is typically up.

Turn a plant upside down and it will reorient itself, albeit in plant time (slowly), so that its growth proceeds in the proper directions. The aerial roots of mangrove and banyan trees always grow down, even though they start out several meters in the air.

Plants know where their various parts are: the locations of their branches, at what angles, and in relation to each other. Growing tendrils have a pretty good idea of where they are going.

Plants alter their shape to compensate for previous misfortune. A tree left leaning from a storm may drop limbs, or grow exclusively on one side to regain balance, either by extending existing limbs in the desired direction, and/or adding new limbs where desired.

Proprioception, whether in plants or animals, comes not from a single sense organ, but by integration of a wide variety of inputs. The timing of when a wind gust hits various plant parts carries with it positional information. But the breeze is merely confirmation of what a plant already knows.

 Wood Sorrel

Common wood sorrel is a small flowering plant native to England and neighboring Europe. Other species of wood sorrel are found in North and South America, and Australia.

The plant has delicate lobed leaves which resemble clover. The leaves droop and close as evening draws near, entirely shutting up at night. Many other plants tuck in to sleep at night.

Wood sorrel foretells wet weather by protectively bringing its leaves in, closing up when raindrops fall. The leaves are also sensitive to temperature and touch.

Wood sorrel also avoid too much Sun. When the light gets too bright, leaves close as if it were raining.

Plant Stress

Excellent but complex examples as to plants’ proficiency with their own physiology, chemistry, and physics come via their responses to various stresses – responses which are invoked by attuned decision-making. Extremes in water availability and temperature illustrate.

Drought

Plants reach deep below surface soil to take water up into their shoots and leaves. Through a process called hydraulic lift, plants also leak water into the bone-dry surface soil to release nutrients and stir microbial activity critical to the plants’ survival. Microbes recycle the nutrient building blocks plants need to grow. ~ American biologist John Stark

Drought – prolonged water deprivation – initiates a cascade of acclimations. Initially, leaf area allocation and activity change.

First, leaf expansion halts. Smaller leaves, with less surface area, lose less water.

If plants become stressed after a substantial leaf area has developed, the older leaves are sacrificed first. Younger leaves remain and may even become more active.

One common response to drought is to slather more wax on the leaves, to reduce transpiration. This produces modest results, as cuticular transpiration accounts for only 5–10 % of total water loss.

Some desert plants take leaf area adjustment to an extreme: they lose all their leaves during a drought. Plants get most of their moisture through their roots.

There is a balancing act in the relations between root and shoot systems. A shoot grows until its water supply is limited. A root grows until its supply of photosynthetic product is limited.

This balance shifts during drought to root extension, to dig deeper for moisture. In well-watered soils, root systems tend to be shallow. Plants send their roots deeper as drought takes hold.

That said, different plant families have different strategies to deal with drought. Grasses suppress root growth as drought takes hold. This response allows the plant to slow water extraction from the soil, treating the residual moisture as a reserve. This root austerity is a temporary measure. When the soil moisture level rises, root growth resumes.

As soil dries, its water potential and matric potential turn negative. So, plants adjust their internal chemistry to absorb more water.

Water potential is the tendency of water to move from one area to another due to osmosis, gravity, pressure, or matrix effects, such as surface tension. Matric potential summarizes the adhesive intermolecular forces that water has for solid particles – in other words, water’s cling to things.

Plants can continue to absorb water only if their water potential remains less than the water source, and the matric potential stays favorable. So, they adjust: accumulating solutes and lowering the osmotic potential of cell sap.

Adjustments in internal solutes (increases in sugars, organic acids, and ions (particularly potassium)) are small but helpful. Shifting water around within cells improves water potential. The 2 actions dovetail to keep cells functioning.

Having experienced drought conditions, plants learn which responses are most effective, thereby letting them better deal with, and more quickly recover from, later dehydration events.

Waterlogging

Waterlogging is an entirely different scenario from drought. Essentially, the problem is anaerobiosis: oxygen starvation (hypoxia).

Shortly after soil is flooded, the respiration of roots and microorganisms depletes the remnant oxygen and the environment becomes hypoxic (i.e., oxygen levels limit mitochondrial respiration and later anoxic (i.e., respiration is completely inhibited). ~ Argentinian botanist Gustavo Gabriel Striker

The major effect of waterlogging is a decrease in gaseous interchange between the atmosphere and soil. This interchange depends upon the rate of biochemical reactions. These reactions are affected by temperature, the concentration of organic substances, and the speed at which each gas moves.

Oxygen diffuses 10,000 faster atmospherically than it does in liquid, and its concentration is 30 times less in water than air. Further, concentration decreases with depth.

To grow, roots respire fulsome amounts of oxygen. If the soil becomes waterlogged, roots can’t breathe, as oxygen diffuses poorly in water. This reduces root water permeability in many plants. Thus, plants growing in flooded soils can suffer reduced water content, and their leaves wilt. The paradox of waterlogging is that it starves plant roots of water.

If a soil is well aerated, oxidized states dominate. In poorly aerated soils, redox reactions are reduced.

In a well-drained soil, plant roots and aerobic microbes happily practice aerobic respiration. In waterlogged soil, anaerobic bacteria replace aerobic ones. CO2 is reduced to methane by methanogenic bacteria.

Even worse, toxic substances build up because of incomplete breakdown of organic substances in waterlogged soil. Organic acids, alcohols, and ethylene, a plant growth regulator, all accumulate under waterlogging.

The primary acclimation to waterlogged conditions, particularly the lowered oxygen in the soil, is in roots developing aerenchyma, which are channels that allow gas exchange between root and shoot. Aerenchyma are naturally formed as part of the development in wetland plants; a process termed schizogeny. For other plants, under the stress of waterlogging, lysigeny is the solution: cells selectively die to produce spaces for gas pathways.

In a transduction not well understood, hypoxia increases ethylene biosynthesis, which controls aerenchyma development. Ethylene also induces other adaptive physiological responses to anaerobiosis, including rapid underwater extension of shoots, as well quickly developing adventitious roots near the soil surface. Both adaptations increase the chances of a plant finding an area with higher oxygen concentrations.

Hydrophytes

A hydrophyte is a plant adapted to living in water-saturated soil or in water. Water lilies, lotus, and water hyacinth are exemplary hydrophytes.

One problem avoided is getting enough water. But there are challenges.

Gas exchange is one difficulty in living a hydrophytic life. Another, for submerged plants, is getting enough light for photosynthesis.

Leaf modifications in hydrophytes enhance light absorption and gas exchange. Water protects immersed leaves from too bright sunlight. Getting enough light is the issue.

Hydrophytes often have large, thin leaves that float. The grand, circular leaves of water lilies are exemplary.

Epidermal leaf cells frequently contain photosynthetic chloroplasts, with ground tissue modified for storage. These are quite unusual adaptations for plants. Hydrophyte leaves and stems commonly contain considerable aerenchyma, facilitating gas exchange.

Mangroves are halophytes, as they live in seawater. In the black mangrove, aerenchyma-filled roots – pneumatophores – stick up in the air from the soil, acting as snorkels, to let gases diffuse into submerged roots. Pneumatophores are most pronounced in plants that live where the water table fluctuates widely.

As plant tissues are denser than water, and so would naturally sink, the leaves and stems of hydrophytes often have adjustable gas chambers to keep a plant buoyant. These chambers facilitate gas exchange, bringing oxygen down to submerged parts.

The inner tissue (endodermis) of hydrophyte stems and leaves channel the flow of water, keeping it confined to the xylem.

Hydrophytes often exhibit leaf dimorphism: submerged leaves are different in shape and structure. Floating leaves have stomata only on their upper surface, while submerged leaves have none.

Leaf shapes are optimized for their immediate environment. Aquatic buttercups are exemplary. Their floating leaves are large and flat, while their submerged leaves are finely dissected and lacelike. This delicate-looking structure maximizes the surface area able to take in circulating nutrients.

Submerged large leaves would put enormous pressure on a plant. Fine or thin leaves underwater allow current to flow through.

Hydrophytes do not need much, if any, cuticle to prevent tissues from drying out. Lacking a cuticle allows nutrients to be taken in over the entire submerged surface.

Minerals are still taken up by roots, if the aquatic plant has roots. As there is no transpiration under water, ions are shoved through the xylem by root pressure.

 Rootless Duckweed

At 1 millimeter, the rootless duckweed is the smallest vascular plant. This rootless native of Africa, Europe, and parts of Asia lives in quiet water bodies, such as ponds.

The rootless duckweed is a mixotroph: producing its own energy via photosynthesis or absorbing it from the environment as dissolved carbon. It multiplies via vegetative reproduction.

The rootless duckweed is highly nutritious. Its greenery is 40% protein, and its turion (overwintering bud) is 40% starch. The rootless duckweed has numerous important amino acids, and copious amounts of dietary minerals and trace elements, including calcium, magnesium, zinc, and vitamin B12.

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Thanks to numerous adaptations and variable responses, plants can survive over a wide range of temperatures. Most crucially, a plant can differentiate between hot and cold. Sensing temperature is essential to optimizing response.

Cold

Just as knowing when to flower is important, so too the need to anticipate the chill of winter. The pathways that protect plants from freezing that were inactivated in spring are prepared in autumn for the arrival of crisp weather. Seasonal sense and activity optimize allocation of resources.

Plants that move in response to temperature are thermonastic. The rhododendron evolved in the mountains where low temperatures are common. On a temperate day, a plant’s leaves are outstretched to soak up the Sun. But when the temperature drops below freezing, leaves curl inwards and roll up. Each leaf then droops to reduce the risk of frost damage.

Cooling slows the rate of photosynthesis and has marked effects on respiration. Protein synthesis is also inhibited.

Tropical plants are especially sensitive to chilling temperatures of 10–15 °C. Cold-sensitive plants have higher levels of saturated fatty acids in their cell membranes. Their membranes solidify more quickly when it turns cold than more tolerant plants.

Generally, highly unsaturated fatty acids, which are important in maintaining membrane fluidity, predominate in plants acclimated to cold climates.

Some plants avoid the worst of the cold by hiding from it. Belowground rhizomes, roots, and tubers are less susceptible to freezing than parts aboveground, which are sacrificed as necessary.

Soil temperature falls more slowly than air temperature. Subterranean plant parts have higher inbuilt frost resistance, as these organs suffer less heat loss.

Alpine and arctic plants get some protection from frost by a layer of snow. Nonetheless, these plants minimize aboveground presence.

The stress from freezing takes more of a toll on the intercellular spaces within a plant than it does on the cells themselves. Plants cells accumulate low-weight organic solutes, such as sugars and amino acids, to lower their freezing point. Such solutes only lower the freezing point a few degrees, but they also protect enzymes from dehydration.

Ice readily forms from particulate nuclei around which ice crystals materialize. Pure water freezes at –40 °C, not 0 °C, which is the transition point for normal water.

Some plants resist freezing by supercooling. Select tissues in cold-hardy plants seem as if they contain pure water. Solute content is largely absent. Such regions can be chilled to minus 38 °C before ice forms.

Such protection is particularly important for dormant buds and the xylem of woody plants during winter. While many cells suffer intercellular ice formation and dehydration, some tissues survive by supercooling.

Some trees grow in habitats that drop below –40 °C in the winter. The above tricks to reduce freezing point don’t work. Instead, intracellular freezing is prevented by withdrawing the cell’s water to the apoplast: the diffusional space outside the plasma membrane. Only thin layers of water molecules are left to protect macromolecules. In effect, plant tissues hibernate freeze-dried.

The temperature rarely drops below freezing suddenly in Nature. Usually, autumn ambient temperature gradually decreases for weeks before the first freeze.

During that time, plants anticipate future freezing. Only 1 or 2 days of near-freezing temperatures are enough to bring about acclimation.

Innumerable minute chemical changes, notably in proteins and polypeptides, create antifreeze properties. Metabolic changes for cold acclimation are epigenetically controlled.

Plants learn how to manage the stress of being cold. Acclimated tolerance improves as a plant matures.

Heat

Plants avoid overheating by accelerating leaf transpiration: a quite effective way to keep cool. The trade-off is that an increase in water loss risks dehydration.

Some plants reduce overheating by turning their leaves away from the Sun. Soybeans and other legumes, as well as mallow family plants, including hibiscus, cotton, cacao, and okra, practice this paraheliotropism. Grasses roll their leaves up to minimize exposure to sunlight.

Many desert plants are xerophytic: adapted to survive in an environment lacking water. The Joshua tree is an exemplary xerophyte.

Xerophytes conserve water by using CAM photosynthesis, which considerably reduces water loss by, among other things, letting stomata (leaf pores) be closed during the day. These measures greatly decrease evaporative cooling, affording greater tolerance to high temperatures.

Cacti illustrate several techniques to avoid overheating. Cacti are covered in dense, reflective spines. Much incident radiation is reflected, and airflow is greatly reduced, lessening water loss. The disadvantage is that reflecting light can decrease photosynthesis.

Starvation occurs when respiration rate exceeds photosynthesis. Heat-adapted plants have much higher temperature compensation points. Adapted plants may also have more efficient photosynthesis at high temperature due to the greater thermal stability of CAM photosynthesis.

Tropical rainforests are hot, but this environment is less of a challenge, as humidity is high, and shading helps prevent excessive heat build-up.

Like freezing resistance, plants learn how to micromanage themselves to accommodate hot weather. Plant tissues sense temperature and induce a variety of intracellular chemical and physiological changes to improve heat tolerance.

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Whether from pest, disease, or adverse environmental conditions, stress takes its toll. Plants learn from the experience and pass their knowledge and acquired immunities to their seedlings. Whence plant evolution proceeds.

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Whether from pest, disease, or adverse environmental conditions, stress takes its toll. Plants learn from the experience and pass their knowledge and acquired immunities to their seedlings. Whence plant evolution proceeds.

Plant Strategies

Plants can be quite strategic. ~ American ecologist Lars Hedin

Plants have strategies to further their own growth, fend off predators and parasites, heal wounds, and establish territories.

Growth and defense are energetically conflicting goals. Defense against predation is essential only at certain times.

Spending energy on defensive measures limits growth potential, but defensive strength cannot be suddenly amassed. So, the trade-off between growth and defense requires an energy budget. At the physical level, hormones that control growth, termed gibberellins, and those that muster defense, termed jasmonates, conference to decide how to allocate the plant’s energy resources. These hormones are merely physical correlates to the mental and life-energy gyre (lengyre) in respectively deciding and effecting strategies.

Plants have to prioritize. ~ Chinese botanist Sheng Yang He

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The forgoing withstanding, some plants are invigorated from suffering hard times: rebounding with vigor.

Plants can benefit from being eaten because they respond by overcompensating, ultimately achieving greater fitness.
~ American biologists Ken Paige & Thomas Whitham

Scarlet gilia is American western wildflower which grows steadily from seed. Scarlet gilia stands tall among sagebrush on mountainsides, its brilliant red trumpet flowers blazing. Anything that doesn’t kill it makes it stronger. There are other plants like it, including some mustards.

Most plants respond to damage with a process called endoreduplication, in which a cell duplicates its genome without splitting into 2. Endoreduplication gives a plant larger cells with more energy factories (mitochondria). Many damaged plants go for minimal levels of endoreduplication, but overcompensators go into overdrive with the process.

In practice, defense and regrowth actually go hand-in-hand because the genetics of defense and regrowth are similar; like it or not, theory be darned. ~ American biologist Josh Banta

Growth

A seed germinates to produce a shoot, which progresses through a juvenile phase to grow into an adult. Then it is time to bring forth the next generation.

Vegetative phase changes are prompted by a sweet sense of certainty. The level of sugar provided by photosynthesis signals to a plant its growth status and triggers developmental changes.

Plants have evolved complex sensory and regulatory systems that allow them to modulate their growth in response to ever-changing conditions. ~ Daniel Chamovitz

Growth is a decision-laden process. A plant must decide how best to allocate its resources amid an incredible diversity of options: root, stem, leaf, or bark growth.

Learning the lay of the land, plants get more ambitious as they grow. In the maturation process, tentative small branches and leaves are forsaken for larger constructions. A plant grows surer of itself as it grows.

Plants in dense vegetation perceive their neighbors primarily through changes in light quality. ~ Dutch botanist Mieke de Wit et al

Plants know that competition is approaching based on the precise color of the light that reaches their leaves. In response they accelerate their growth. The closer the competition comes, the harder a plant pushes itself to get high.

Plants adapt their growth and development to changes in the environment, as well as exhibit considerable plasticity in their functional response. Plant fitness depends on plasticity. ~ Dutch biologist Ben Scheres & Dutch terrestrial ecologist Wim van der Putten

Cells

Plant cell-wall growth is the underlying mechanism by which really small seedlings can grow to really large trees. It’s the cellular basis for the way leaves expand. ~ American biologist Daniel Cosgrove

Animals and their cells are typically motile. In contrast, plant cells are immobile. Plant development and growth depend upon cell expansion rather than cell migration.

Cell expansion is a basic aspect of plant growth. Its rate and direction are dynamically regulated, adapting to perceived internal and environmental conditions.

Growth occurs when water moves into a cell and inflates it. The speed of most growth responses is determined by the rate of water movement in a tissue. ~ American botanist Wendy Silk

The scaffolding for a plant cell is provided by its cytoskeleton, which comprises an array of tubule protein fibers (microtubules). Along with structural support, microtubules guide oriented deposition of cell wall components.

Along with augmentation, microtubular arrays move during cell growth in the expansion direction, which is determined by exposure to blue light.

To maintain appropriate cell expansion, plant cells fine-tune signaling pathways. This is an ongoing interpretive exercise, based upon an extensive information network.

To properly distribute nutrients and other needed molecules, the contents of plants cells are continuously mixed. Cytoplasmic streaming – the directed flow of cytosol through plant cells – is the distribution process.

The stirring of cytosol is achieved by the motor protein myosin xi. Molecular motors like myosin can move the surface of a specific substrate.

Myosin xi slides along actin filaments in the cell’s skeleton. The velocity of myosin xi movement determines the rate of cytoplasmic streaming, which ordains how quickly plant cells can expand.

The pace of plant cell expansion is a key determinant in plant size. Accordingly, the tempo of myosin xi is a critical factor in plant growth rate, and limits how large a plant may become.

 Auxin

Auxin regulates nearly all aspects of plant development and behavior and impinges on a great variety of responses involving cell polarization, expansion, division and differentiation. ~ Chinese botanist Tongda Xu et al

Auxin is a class of hormones that are instrumental in coordinating plant development processes. The local concentration of auxin affects the growth direction of stems, roots, and shoots. Growth goes where auxin flows.

In sensing gravity, plants literally know what’s up. With proper gravitas, auxin directs tree branches to grow at a specific angle.

Auxin also factors in the ripening of fruit, the clinging of climbers, and numerous other behaviors. Auxin works epigenetically in being instrumental to selectively activating genes.
Leaves

Leaves originate from stem cells located at the shoot apical meristem. The meristem is shielded from the environment by older leaves. Light acts as a morphogenic signal that controls leaf initiation and stabilizes leaf positioning. ~ Japanese botanist Saiko Yoshida et al

Leaf shape arises through feedback between early patterns of oriented growth and tissue deformation. ~ English botanist Enrico Coen

Photosynthetic leaves may appear in a variety of sizes, determined by risk analysis related to available resources. Growth and maintenance follow a return-on-investment model: how much to invest given potential returns.

Because chlorophyll absorbs the red light in sunshine, light that passes through a leaf is enriched in the far-red end of the light spectrum. Many plants sense this spectral quality and use it to change their growth pattern. Shade avoidance results in longer, thinner stems, reduced leaf production, and positioning to put new leaves into the brightest possible place.

The recyclable chemical contents of a leaf are taken back into the plant if the leaf is not earning enough energy return. This happens to leaves in the shade, or when a leaf is aged enough that estimated cost of repair outweighs potential return.

A leaf shrivels and browns during the investment recall process and is discarded when that process completes. In contrast, autumn leaves that turn yellow and red are loaded with protective chemicals and dropped as winter preparation: to safeguard a tree and its seedling offspring.

Photoperiodism is the term for plant responses to changes in the length of days and nights. Photoperiodism affects many plant processes, including bud dormancy and the formation of storage organs. The timing of flowering is typically a photoperiodic phenomenon.

A flowering plant generates many different organs, such as leaves, petals, and stamens. Each organ has its own specific shape and function, yet all emerge from a common cellular base. This is possible because of intricate maps that orient growth and make decisions based upon voluminous information about a plant’s state and external environmental conditions.

Roots typically provide a plant with its water supply. In species that live in areas that are foggy but dry, leaves assist water uptake. The leaves of montane cloud forest trees – constantly bathed in moisture-rich clouds – also soak in water, especially during the annual dry season, when there are clouds but little rain.

Roots

A root is a complex assemblage. Besides sensing its environment, including humidity, light level, gravity, oxygen, and nutrients, a root cap protects the root as it navigates through the soil. Behind this is the meristem, a region of rapidly growing cells. A root consolidates and processes the information it has in the transition zone, deciding how to proceed. The transition zone houses the mind of a root. Cells in the elongation zone grow in length, letting a root extend and bend.

Growing roots show complex patterns of behaviour such as decision-making, sensory-motoric circuits, search and escape movements, as well as self–non-self and kin recognition. ~ German botanist František Baluška et al

Plants develop extremely complex root systems which colonize large soil areas. Growing roots show coordinated group behaviour that allows them to exploit the soil resources optimally. Roots enjoy a rich ‘social’ life at the individual plant level and they continuously solve cognitive problems. ~ botanists František Baluška, Simcha Lev-Yadun, & Stefano Mancuso

Each root tip has an independent intelligence, with a memory of its life experience. Ongoing electrical and chemical communication among root apices provide for a swarm intelligence network, affording collective decision-making that coordinates navigation and growth patterns, as well as waging territorial war against rival root systems.

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Plants require 15 essential elements. All but a few are obtained as ions dissolved in soil water.

Many of these elemental nutrients are required in minute quantities. Even so, their absence limits growth. For example, peat soils are wholly organic. Their lack of mineral reserve is most telling in inducing copper deficiency.

For most soils, the 2 elements that most bound plant performance are nitrogen and phosphorus.

Plants actively cultivate and then extract nutrients from symbiotic microbes. ~ American botanist James White Jr.

Almost all the nitrogen in soil is recycled. Its initial deposit is from dead organisms. To be usable by plants, nitrogen must be mineralized: turned from organic into inorganic form. This work is done by decomposers, whose performance depends upon soil conditions.

Whereas the nitrogen problem is one of availability, not scarcity, phosphorus presents an obverse challenge. Phosphorus is a rare element. If available, its uptake by roots is relatively uneventful.

Rocks contain less than 1% phosphorus, yet their weathering is the primary natural source of introducing phosphate into organically usable form.

Like nitrogen, most phosphorus is recycled in the substrate where life exists, whether in the water or on land. Phosphate (PO43–) is the biologically employed form of phosphorus. Whereas phosphate quickly moves through plants and animals, the phosphorus cycle moves very slowly in the ocean and soil.

 Rhizosphere

The realm of soil under plant management is the rhizosphere. Plants can alter soil conditions in many ways. Plants make decisions as to what actions to take, depending upon their needs and perceived possibilities.

Dropping pH affords some increase in nutrient uptake, particularly phosphorus and nitrogen. This stratagem works by altering the ion balances in the rhizosphere.

To snag phosphorous, fava beans rapidly acidify their rhizosphere. They can drop pH 2 points in 6 hours.

Iron-deficient plants secrete siderophores: compounds which chelate iron and render it soluble. Microflora also have this knack. It may be that plant siderophores are meant mostly as an invitation to microbes, by providing an environment conducive to iron absorption, as microflora are much more efficient at providing soluble iron which plants can absorb.

To greatest effect, plants generally follow the adage of “a friend in need is a friend indeed,” by turning their rhizopheres into havens for the fungi and bacteria that can assist them in elemental nutrient uptake, by breaking minerals down into absorbable form. This often involves feeding the microflora: exuding sugary metabolites as solicitation and as payment for work well done.

Facing a deficiency of soil-based resources, a plant shifts its growth pattern, favoring root over shoot. Since nutrient uptake depends largely upon the geometry of the root system, the greatest probable return-on-investment (ROI) comes from maximizing root length. This favors fine roots, since they achieve the greatest root length for given weight and can be quickly extended.

The problem is more complex than simply spreading out. The root topology problem comes in architecting a root system that optimizes coverage without introducing competition between old roots and new and do so with minimal investment while maximizing ROI. Plants solve this 3-dimensional graphing conundrum with aplomb.

Certain branching patterns are more expensive to construct but are more efficient at exploring soil. A fractal herringbone pattern consists of axis roots with finer laterals running from the axis, in successive iterations.

Thicker axis roots come at a greater cost than finer ones. But the extensions from such fractal herringbone links are not proximate, and so do not compete for nutrient ions.

Roots scrounging for nutrients brings home the favorable economics of outsourcing. Symbiotic fungi (mycorrhiza) are much more efficient producers of inorganic supplies. For a given investment of resources, fungal filaments (hyphae) are at least 100 times more efficient than building roots; hence the widespread employment of certain fungi by plants.

Another issue is patchiness. Available sunlight is a gradient. Suboptimal return from leaves in the shade is overcome by shoot growth to take the leaves to a sunnier spot, or abandonment of the endeavor there, if the prospect for better light in the immediate vicinity is dim.

In contrast, soil often has a decided heterogeneity of barren areas and nutrient hotspots. When roots encounter a nutrient-rich patch, they intuitively proliferate.

Not all plants are so exacting in their extracting. The species that vigorously respond to nutrient-rich areas with precise root placement are small and slow-growing. Those that largely ignore fertile clumps and protrude roots more single-mindedly are large, fast growing, and highly competitive. Their strategy is to cover ground and crowd out any potential competitors.

The economics of root proliferation to exploit fertile spots is complex. To be advantageous, nutrient uptake from increased root density has to justify the cost of new roots.

If a rich patch taps out quickly, a small gain is had at the cost of maintaining roots there. Fine roots are easily forgone as a small investment.

The thicker the roots, the less likely that their construction cost will be recouped if potential return has been misjudged. Coarse-rooted plant species are less enthused about exploiting nutrient hotspots than species adept at fine rooting. Across global biomes, plants have consistently evolved thinner roots to improve their productivity.

Plants evolving thinner roots enabled them to markedly improve their efficiency of soil exploration per unit of carbon invested and to reduce their dependence on symbiotic mycorrhizal fungi. ~ Chinese botanist Zeqing Ma et al

Plants alter the spatial distribution of their roots based upon resource patchiness and competition from other plants; decisions based upon reward/risk analysis.

Roots are very adaptive at modifying growth throughout the root system to concentrate their efforts in the areas that are the most profitable. ~ English botanist Angela Hodge

 Kwongan Root Strategies

Kwongan is an arid biome in southwestern Australia. Despite having some of the most infertile soils in the world, this bushland has exceptionally rich plant biodiversity.

Plants that grow in land with infertile soil all adopt the same aboveground strategy: producing tough leaves that survive for years. Below ground is a much different story.

The root systems of Kwongan plants adopt divergent strategies for obtaining needed nutrients. Some form symbiotic relations with fungi or bacteria. Others capture insects and digest them. A sizeable number of species exude metabolites that increase nutrient availability from the impoverished soil.

Plants living next to one another can use completely different strategies and have just as much success. ~ Canadian plant ecologist Etienne Laliberté

Seasonal Sense

Different plant parts and growth facets operate on differently timed cycles. Plants have several types of biological clocks, related to circadian cycles and rhythmical processes. This lets them anticipate environmental and biological events that occur at precise times of the day. Photosynthesis, fragrance emission, and blooming are all time regulated. Plants in a community often bloom at the same time to optimize the benefits of interbreeding.

A fungal pathogen attacks a rockcress plant, giving it a downy mildew disease: weakening the plant, and giving it a fuzzy mildew coating. The pathogen forms spores at night, releasing them at dawn.

Rockcress fights the pathogen by immunizing itself from the evening onward, with peak resistance at dawn. During the day, the genes that express protection are inactive.

Numerous plant processes are seasonally symphonic. Tree leaves fall at the same time as buds go dormant, cambial activity (cell growth) decreases, while contemporaneously preparation for cold tolerance steps up.

Yet the bioclocks involved in these seemingly synchronous activities may be quite different, or responses staggered. Bud dormancy is triggered by shortening days, as perceived earlier by the phytochrome in leaves, and pattern recognized.

Leaves fall after considerable preparation: retrieving recyclable nutrients and loading them with secondary metabolites. Cambial activity is regulated by a complex confluence of resource availability and perceived environmental conditions, which are synchronized with and summarized by bioclock indications.

Chemical reactions are highly temperature dependent. Chemical reaction rate doubles with a 10 °C increase in temperature. Plants compensate accordingly: planning their activities and metabolite productions based upon anticipated weather patterns.

Temperatures just above freezing (0–5 °C) typically retard plants’ biological clocks for the duration of cold exposure. After prolonged chill, warming resets clocks.

 Flowering

The transition to flowering is complex and involves the convergence of multiple signals. ~ Swedish botanist Jonas Danielson & German botanist Wolf Frommer

For an angiosperm, flowering largely determines a plant’s reproductive success. Multiple criteria must be met for a plant to decide to flower. Unless and until a plant feels that it is healthy and ready, it will not attempt flowering.

Many flowering plants do not produce blooms until they have experienced the cold of winter – a condition called vernalization. Winter wheat and various fruit trees, including cherry, peach, and orange, are exemplary. Biennial plants, such as sugar beet, cabbage, celery, and carrots, need chilling for 2nd-year flowering buds to develop.

The chilling experience is registered epigenetically. Vernalization is but one example of innumerable instances where plants keep track of events and remember the rhythms of their environment. Vernalization also exemplifies the innate wisdom in plants to conserve their resources until they understand their situation. Experience confers confidence.

Defenses

Because plants cannot run away from danger, they have evolved defenses against pathogens and herbivores that rival, and even exceed, the sophistication of many animal immune systems. ~ American ethologist Andrew Zink & Chinese botanist Zheng-Hui He

Plants possess a repertoire of defenses and healing remedies. Plants make carefully determinative decisions about investing in defense instead of growth.

Plants respond to their environment with potent defenses. ~ American biologist John Orrock

By recognizing signature molecules of microbial malevolence, plants are actively aware of an infection and consciously decide how to deal with it. Among other options, they might decide to sacrifice a region around the infected area to prevent its spread.

Plants have an innate ability to recognise potentially harmful bacteria and launch an immune response. ~ French botanist Cyril Zipfel

Infection stresses a plant. This triggers a sophisticated series of defense mechanisms, stimulating various hormones that trigger alterations in gene expression networks.

Plants can actively create an ecology of assistance from their own and other species. Certain bacteria that live at the root of a plant may warn of an impending attack, suggesting to the plant to close its leaf stomata. These pores not only provide an opening for air and water, but also pathogens on the prowl. Helpful bacteria use the plant’s signaling pathways from root to shoot to alert a plant that tiny trouble is afoot. The plant conveys its thanks by cultivating more of the sentinel bacteria, at least until the crisis passes.

Wounds

Plants respond within minutes to stresses such as wounding with both local and system-wide reactions that prime nondamaged regions to mount defenses. ~ Japanese molecular biologist Masatsugu Toyota et al

In a world with marauding animals, plants wounds are unavoidable. Treading and grazing by beasts brings a barbaric beating.

Damaged and dying cells are more easily colonized by pathogenic microbes than healthy cells. An open wound provides a point of entry.

Wounding sparks production of reactive oxygen species: the same weapon that animal immune systems use to kill invaders. A wounded plant then seals the wound with lignin or another hydrophobic polymer.

Further protection is provided by producing polyphenols in punked cells. A polyphenol is a cross-linked phenolic compound, synthesized when a cell is exposed to an oxidizing environment, which does not occur unless the cell wall has been breached.

Plant polyphenols act as an antiseptic. The browning of bruised apples or potatoes comes from polyphenol production.

Plants engage various repair systems and initiate chemical responses to thwart further damage. Signaling molecules course through roots, stalks, and limbs, rallying cellular troops: the genome is enlisted to manufacture defense-related proteins.

Border Building

Besides the pure play of producing localized poison, an arrow in the quiver of plant defense is a sacrifice gambit. Plants fight diseases by programmed cell suicide in the immediate area of invasion, to starve and poison the pathogen. Cell death prevents infestation by biotrophic pathogens, which require living cells to establish a successful colony.

Plants will load dying cells with high concentrations of antimicrobial compounds, building a poisonous border to healthy tissue. This dead-cell border also prevents the symplastic spread of effector molecules or toxins introduced by the pathogen to further its invasion.

Self-Protection

Plants are winning mostly. Insects are always trying to catch up. ~ American entomologist Mark Mesche

They are not called bugs for nothing. Insects pester plants more than any other animal. A plant often outsmarts its 6-legged opponent, sometimes with considerable cunning. Goldenrods deterring egg-laying on their leaves by gallflies, their nemesis, through specialized toxins, is one of many such examples.

 Inciting Cannibalism

At the end of the day, somebody gets eaten. ~ John Orrock

When the supply of their favorite vegetable is running low, herbivorous insects start eating their competition. Plants know this and produce metabolites that engender pest-on-pest predation.

From the plant’s perspective, this is a pretty sweet outcome; turning herbivores on each other. ~ John Orrock

When being unrelentingly attacked by caterpillars, tomato plants produce a compound that induces the pests to lay off their leaves and turn into cannibals. The cost to the plant of mounting this defense is considerable, but it is much better than being eaten alive.

Plants strike a balance and decide if the attack is serious enough to activate the defenses. ~ John Orrock

 Beetle Juice

Symbionts can influence plant–insect interactions. ~ Korean American entomologist Seung Ho Chung et al

A plant can tell what is eating at it. The defense against gnawing insects proceeds along a pathway that creates chemicals to stop the digestion and growth of the pest.

This pathway is mediated by jasmonic acid. If attacked by a microbe, a different, mutually exclusive pathway is employed which is mediated by salicylic acid.

Potato bug larvae have friends that get them past the plant defense system. Before chowing down on a potato or tomato plant, bug gut bacteria are brought up to fool the plant as to what is going on.

These beetles don’t have salivary glands. They regurgitate some stomach contents onto leaves to begin digestion. These secretions have the bacterial tricksters.

The bacteria make the plant think it is being attacked by microbes instead of an insect. While some symbionts are sacrificed in the deception, the beetle dines and keeps the microbiome within well-fed.

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Plants have strategies to preemptively protect themselves, and to do so efficiently. Several of these strategies are contemplated decisions to wisely use resources and protect against predators.

Plants tightly control the onset and amplitude of potent immune responses to pathogens for optimal growth and development. ~ American botanist Walter Gassmann et al

To avoid predators on the ground, the Australian brushtail possum lives in the trees. Eucalyptus leaves are a mainstay of the possum’s diet.

Once a tree realizes that its leaves are being munched, it quickly pumps eucalyptol, a possum preventative, into its leaves, to get the possum to forage somewhere else. Eucalyptol also acts as an insecticide. A eucalyptus tree controls its production and application of eucalyptol to meet its needs.

Many plants have subtle mechanisms that promote production of protective secondary metabolites only when needed. They know what time it is, and the time of day that common pests rouse themselves to bring ruin. In anticipation, plants ramp their defenses in likely locations of assault.

New leaves begin to grow only when stimulated by light: the prospect of profit. The setup involves a subtle subterfuge.

The region at the top of the plant where new leaves emerge (the shoot apical meristem) is sheltered by existing leaves. The covering hides the new shoots from predation by herbivores, as this new growth has yet to develop its own chemical protection. Plant development is a timed investment, intelligently made in concert with proper conditions.

Once they appear, the chemically unprotected shoots and leaves of early spring are highly prized by herbivores. Besides hiding behind veteran leaves, some plants color new growth in ways that disguise its digestibility. Plants somehow understand the inner workings of the vision system of herbivores.

Roots, particularly those subject to subterranean grazing by parasitic fungi and bacteria, commonly contain the highest concentrations of metabolites designed to ward off microbial predation. It is no coincidence that ancient Greek herbalists, who knew where the metabolite medicines were stored, were called “root diggers.”

Variations in the distribution of secondary compounds extend beyond anatomy and season. Plants are aware of predators.

After gypsy moths have defoliated oak trees, new leaves emerge with a higher tannin concentration. Tannin molecules bind with plant tissue proteins, rendering otherwise tasty bits indigestible to plant-eating insects. Such leaves are also tougher, with lower water content. The combination retards development of gypsy moth caterpillars and the growth of adults.

Besides inciting local defense responses, wounding provokes plant-wide resistance to further herbivore attack. A plant increases production of protease inhibitors that resist insect proteases, and so thwart predator digestion. Proteases are enzymes used by all organisms to facilitate digestion, particularly breaking apart the peptide bonds that hold proteins together.

Potato plants damaged by insects ramp the making of protease inhibitors and other chemicals that disrupt predator digestion and stunts its growth. The first damaged leaf signals others to crank up protection production.

Plants determine who is attacking them. They’re even capable of distinguishing between a native and an exotic herbivore. ~ Dutch botanist Nicole van Dam

 Calling for Help

Plants respond to insect herbivory with the production of volatiles that attract carnivorous enemies of the herbivores, a phenomenon called indirect defence or ‘plants crying for help’. ~ Dutch entomologist Marcel Dicke

Sitka willow leaves ravaged by insects lower their nutritional value to dissuade further savaging. The tale of the attack is passed to other willows, who respond likewise.

Numerous plants ring warning bells for other species as well. Sage releases methyl jasmonate, a volatile compound, when its leaves are crushed. This cue stimulates nearby tomato plants to produce protease inhibitors that grapple against grazing insects.

When wounded, some plants scream for help; not just to other plants, but also to insects. The enemy of my enemy is my friend.

When caterpillars or mites bite into stems or leaves, an attacked plant oozes an aroma that acts as a dinner invitation to the insects that prey on the attackers. Responders flock to the attacked plant to dine on what’s eating the plant.

Plants being chewed on by beet armyworms waft volatiles that call to wasps which parasitize armyworm larvae. The call for help with secondary metabolites is not always an unalloyed success.

During daylight hours, beet armyworms assiduously avoid a plant that is fuming mad over being munched. The risk of parasitization is too great. But the scent of plant outrage invokes a different response at night. If not already munching away, armyworms march to a tasty plant in dire straits, as wasps don’t work the night shift.

Egyptian cotton leafworm caterpillars clamber to a corn plant crying for help. When these leafworm caterpillars drop from a plant they become vulnerable to predators and pathogens in the soil, as well as starvation. Competition may be more intense, but the corn SOS scent is a reliable signal of a plant they know they can feed on. Meanwhile, adult leafworm moths can flit to where they like, so they avoid maize that is being mangled, as there are easier pickings elsewhere.

Spore-producing ferns predate seed plants. They do not call for help when attacked by herbivores. But they could. Ferns have the metabolites. They just don’t put them out in a volatile form that signals others. Instead, ferns keep their herbivore repellent within. Such reserve has been successful. Ferns, still abundant today, have been around for 400 million years.

 Tobacco’s Defense

Plants selectively respond to an attack, making considered decisions to bring about the most favorable outcome. Tobacco plants decide what to do based on what is attacking them. If a tobacco plant is assaulted by hornworm caterpillars, it immediately synthesizes deterrents at the attack site, then in more distant parts of the plant, as a preemptive measure against further threat.

An attack trigger releases a first set of compounds that persuade the caterpillar not to lay eggs. A different metabolite release calls for the aid of parasitic wasps that lay their eggs on hornworm caterpillars. (Geocoris pallens are the parasitic wasps that lay their eggs on hornworm caterpillars.) The wasp larvae hatch and eat the hornworm caterpillars in short order.

If something other than a hornworm is attacking, a tobacco plant produces nicotine, a nasty alkaloid.

Nicotine is made in the roots and carried to the leaves. This allows a systemic defense to be mounted even after most of the upper parts of the plant have been eaten.

For 2 reasons, tobacco plants do not bother with nicotine against hornworms. 1st, nicotine wards off the parasitic wasps needed to hamper hornworm eggs. 2nd, hornworm caterpillars more than tolerate nicotine: they store it, to preclude their own demise by predators that would otherwise find them tasty.

 The Timing of Fruit

Fruits bear the seeds of the next generation. These are irresistible treats to animal agents who spread a plant’s progeny as a time-released activity. Species survival depends upon protection until the seeds are ready to be dispersed.

Signaled by the ripe red of cherries and other berries, birds feast, and then release the seeds far from the parent plant. The bright pigmentation tells the birds that the time has arrived for them to eat fruit.

During its preparation, berry fruit remains hard, green, and starchy; often laden with astringent or bitter compounds, and thus protected against pathogenic microbes and hungry herbivores. Though red holly berries have high levels of bitter tannins that inhibit insects, the compound does not deter birds and squirrels from consuming the fruit and dispersing the seeds. These plants know the fine line between inhibiting pests and soliciting assistance.

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Several plants plant a time bomb for pests. Armyworms, the caterpillar of a phalaenid moth, are fond of daisies. As a countermeasure, daisy leaves are laced with polyacetylenes: highly electrically conducting compounds that become toxic when exposed to sunlight.

Once a worm has digested laced leaves, the polyacetylene within travels to the worm’s surface tissues, catches some UV rays, and turns lethal. The armyworm quickly shrivels up and dies.

While the armyworm lacks armor to protect itself from the daisy bomb, “leaf-rollers” blanket themselves from poisoning. Leaf-rollers is a colloquial name for various moth caterpillars, weevils, and wasps that have a strategic counterplan to photosensitive plant bombs. They shape a leaf into a shelter, letting them munch in the shade of their protective pocket, out of the Sun, and out of sight from potential predators.

Insects have adapted various ways around plant defenses. Sucking insects, such as cicadas and leafhoppers, tap directly into the cells conducting succulent water and sugary sap. Aphids punch past leaf epidermis laden with secondary compounds, and so precisely dine on the fine juices within.

Other insects feed exclusively on nutrient-rich seeds: a source of proteins and fats, and a luxurious diet for many insect larvae.

While some insects feed on a variety of plants, many specialize, identifying favorites by their individual chemistry. The secondary compounds designed to act as a deterrent instead provide an olfactory signal of specificity.

Polyphagous feeders must adapt to digest various compounds from the different plants they eat, while picky oligophagous eaters have adaptively adjusted only to their selected species. Insects adaptively handle the toxins by either disarming them or becoming tolerant to their effects. As with hornworms, some insects employ plant secondary compounds for their own defense, themselves becoming toxic to their predators simply by eating well.

 Immune Systems

Being able to distinguish oneself from others is a fundamental requirement of any immune system. Every plant cell safeguards itself with an immune system.

Plants are constantly interpreting microbial signals from potential pathogens and potential commensals or mutualists. Because plants have no circulating cells dedicated to this task, every plant cell must recognize any microbe as friend, foe, or irrelevant bystander. That tall order is mediated by an innate immune system. ~ American biologists Jeffery Dangl & Marc Nishimura

Plants have pattern-recognition receptors, both outside and within cells, that act as sentries. These receptors individually recognize various viruses, bacteria, fungi, oömycetes, and insects.

Plant guardian proteins are even more diligent defenders when teamed together, which they do. Immunity is a coordinated system.

Plants carry extremely diverse and dynamic repertoires of immune receptors that are interconnected in complex ways. ~ Tunisian botanist Sophien Kamoun et al

New defense proteins proliferate at a faster rate than any other adaptive response that plants have. Plant safety requires a vigilant watch against a profuse number of enemies that keep trying new tactics.

Plant pathogens pursue 1 of 2 strategies. One kills cells and harvests the remains for food. The other is parasitic: setting up housekeeping and reaping regular meals.

A plant has distinct defenses for each type of infection. Cell killers (necrotrophs) are treated with jasmonic acid while the plant does what it can to keep cells alive. Conversely, a plant tries to kill cells infected with parasites (biotrophs).

These different responses are contrary, and so must be judiciously applied. Plants have feedback systems to carefully calibrate appropriate responses.

Via adaptive experience, plants learn how to resist various invasions. Some pathogens manage to outsmart plants by hijacking the very pathways that plants use to protect themselves.

 Damn Spots

Phytopathogens can manipulate plant hormone signaling to access nutrients and counteract defense responses. ~ Chinese botanist Xiao-yu Zheng et al

Pseudomonas syringae is bacterial pathogen that produces brown leaf spots in over 50 different plants. The brown spot bacterium is a biotroph but fools a plant into thinking it is a nectrotroph.

Pseudomonas produces coronatine, which mimics jasmonic acid, and initiates a cascade of molecular activity that mimics a necrotrophic invasion. The plant responds by lowering salicylic acid production to keep the cell alive. Normally, salicylic acid is involved in closing stomata, and acts an essential signal for the cell-killing defense that would be most effective against a biotroph.

By confusing the plant and jamming signals, Pseudomonas keeps the stomata open for further infestation. Meanwhile, it has free rein to reproduce in extracellular spaces.

 Viruses

Plant viruses present a mixed picture. Though little is known of beneficial plant viruses, there are many.

Helpful viruses may confer drought or heat tolerance. Grass that grows in the geyser field in Yellowstone park tolerates heat thanks to a virus that lives in a fungus with which the grass has a symbiotic relationship.

There are 450 species of plant-pathogenic viruses which cause a wide range of diseases. Viruses are most virulent invaders: stealthy pirates with chameleon qualities.

Gaining arrows for their quiver, viruses accumulate RNA during their life cycle with the goal of improving their aim in hitting a chosen target while avoiding defenses thrown against it.

Plants are often able to recover by using their wits. Those infected by viruses may produce new shoots free of viral disease symptoms.

Unlike animals, Nature gives plants a 2nd chance. Acute animal viral infections either end in death or the virus being overwhelmed into defeat: a war where an animal immune system throws everything it has and wins by smothering and flushing an invading virus from the system.

RNA Silencing

Unlike animals, plants rid themselves of viruses through finesse. They do so by robbing a virus of replication ability: silencing the virus’ thrust by silencing its RNA. RNA silencing signals are transmitted between cells and amplified as necessary in a dynamic self-regulated by feedback.

RNA silencing has 3 qualities that make it an effective defense. 1st, the response is specific to the viral gene expression. It has no effect on host-encoded genetic material.

2nd, a response may be amplified and attenuated as necessary. The amplification ability makes it effective even against a rapidly replicating virus.

3rd, the defense maneuver is independent of viral action. Plant signaling may move with or ahead of a virus. A virus cannot escape the effects of silencing by movement.

A plant’s fight against a virus is based upon intimate knowledge of how genetics works: the ability to willfully manipulate at the molecular level. Basically, a plant foils the weapons that a virus uses to invade.

Viruses evolve various stratagems to counteract plant defenses. The primary counter employs proteins which suppress RNA silencing.

Various viruses independently developed different suppression techniques. In several instances, the suppressors were previously employed by viruses for other uses. Viruses adapt the proteins for their new role.

Viral response to attempted RNA silencing by plants is payback. Viral disease symptoms, especially stunting and abnormal development, are the handiwork of viral suppressor proteins.

3 silencing pathways have been identified. All are of an ancient lineage in genetic knowledge. Each pathway is employed in animals, fungi, and plants. Other life forms either did not learn RNA silencing or lost the knowledge during their descent. Only plants employ all 3 as needed.

Mammals can only silence the activity of microRNA by DNA methylation and transcription suppression. That ability does not seem to be applied by mammals as a defense against viruses like it is in plants.

Movement

Plant movements are basic processes that underlie all of plant physiology and growth. ~ American botanist Sarah Wyatt

It is widely believed that plants don’t behave because they don’t move. But plants do move.

The rock-rose Cistus grows near the coast in the Mediterranean region. Pollinated by insects, the rock-rose could risk losing its pollen to blustering sea breezes. Instead, the rock-rose holds its stamens clumped together. Pollen from the anthers are thus prevented from loss owing to the vagaries of the wind. When a pollinator lands on the flower, the stamens respond by opening within a second or 2.

Many flowers have moving stamens which facilitate delivering a load of pollen to a pollinator. The stamens of a moss rose respond within seconds, turning toward the insect that has stimulated it. If both sides have been touched, the stamen remains stationary.

The cornflower is typical of asters, in having stamens which responsively contract to expose the female stigma within. The stigma receives fertilizing pollen.

When stimulated, the anthers that normally hide the stigma draw back, exposing the tip of the stigma. This affords pollination from another cornflower while precluding the stigma from getting pollen from its own flower. A new supply of pollen is then provided to the visitor for the next cornflower it encounters.

Many flowers have perfected pollen delivery by squeezing it out of the anthers onto an alighting insect. Trigger plants turn pollination into a serious contact sport: whacking an arriving insect within 15 milliseconds with a dash of pollen delivered by the flower’s reproductive organs. The pollinator is stunned but unhurt.

Many plants have sexual movements. Moving female sex organs are common.

The monkey-flower has a bloom in which the stigma is divided at its tip into 2 open lips. If either lip is touched, the 2 lips close together, enfolding any pollen which may have been deposited. If the flower has been fertilized, the lips stayed close. Otherwise, the lips part.

The stigma lips of a monkey-flower may take 10 seconds to close. In contrast, a cat’s claw snaps its stigma shut within 2–3 seconds.

 Hopping Horsetail Legs

Horsetail is a living fossil in being a vascular plant that reproduces by spores, when all its relations have since moved onto seeds. But the spores that horsetails sport are not ordinary. Horsetail spores are especially uppity. They have hair-like elaters: moisture-sensitive protrusions.

These little legs are made of 2 layers. One is stiff. The other is a softer, sponge-like material that deflates when dry.

Because the 2 layers are bonded, 1 layer shrinks when the humidity drops, but not the other. This causes a tension that results in a springy leap.

Jumping up in the air lets a spore catch wind currents which may carry it considerable distances; a lively dispersal technique. Elaters are especially effective because winds often pick up with weather changes to lower humidity.

 Moss Spore Shoot

Before vascular plants were bryophytes, which are the typical spore producers. While relatively primitive, they are not without their tricks.

Mosses live low to the ground. Spore dispersal presents a problem.

The first step to solution is to release the spores from stalks sticking up. Then sphagnum moss launches its spores with a twist: popping spores out in a roiling vortex ring that lifts them higher than any straightforward blast ever could.

 Orchids

Orchids have some of the most highly developed pollination movements, typically reliant upon an exquisite sense of touch.

Orchids have a characteristic special petal at the bottom of the flower: a labellum which acts as a landing platform for pollinating insects. In some orchids, the labellum rapidly folds down to entrap the insect. There is a visible opening which the pollinator struggles to get through; in the process delivering its previously gathered pollen to the orchid’s stigma and picking up a fresh pollen coat on the way out.

Many orchids mimic insects of interest, alluring them with a blossom lure. An Australian orchid, Drakaea, has a labellum that looks just a like a certain female wasp. Drakaea also emits the scent of that wasp.

Male wasps try to mate with Drakaea blossoms. A labellum responds by pulling the amorous wasp in and slapping a sticky pollen packet on it. Any previously adhered pollen is collected by the stigma. The wasp gets nothing for its effort but frustration.

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The touch-me-not also quickly responds to tactile stimulus, as does the rambling vine called cat’s claw, which grows in the southern United States. Its cover of luxurious leaves hide long, prickly thorns. When touched, the leaves collapse, exposing its barbs.

Biophytum go one better. Sensing approach, they fold their leaves before an insect lands.

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Tropisms are plant movements in response to environmental conditions. There are several tropisms besides phototropism (light exposure) and photoperiodism (day length), including chemotropism (chemical) and gravitropism (gravity).

Thigmotropism is plant response to touching a solid object. The plasma membranes of epidermal cells distort, creating a conduction of action potential through neighboring cells. Protein-based recognition results in calcium signaling, which eventuates in the appropriate response. The process resembles animals’ sense of touch.

Tendrils and vine stems, such as morning glories, coil when they come upon something. Thigmotropism allows vines to climb over other plants or obstacles, thus improving the prospect for peppy photosynthesis.

 Creeping Dogwood

Creeping dogwood is a slow-growing herbaceous subshrub that forms a carpet-like mat. This dogwood is native to eastern Asia and northern North America.

Each flower has 4 elastic petals that barely hold 4 cocked, filament stamens which protrude from the petals’ embrace. When disturbed, the petals split apart, freeing the stamens. The filaments snap upward, flinging out pollen at high speed. Release takes less than a half millisecond. The pollen sack is flung at 2,400 times the force of gravity. The pollen burst sticks to whatever pollinator may be above it, or into the wind.

 Bladderworts

Bladderworts are the smallest of carnivorous plants, and the most sophisticated. ~ Czech botanist Lubomír Adamec

A water flea bumbles into a little cup in a bladderwort. A touch-sensitive hair-trigger opens a trapdoor, and the flea whooshes into the bladderwort’s stomach.

Bladderwort traps act as an elastic buckle. At just the right pressure, the domelike trap stays shut.

A slight touch springs open a door into the trap, which closes in fractions of a second. No plant has a faster a mechanical mechanism.

 Venus Flytraps

A Venus flytrap sits with its jaw-like leaves open. The leaves are lined on the edged with succulent sugary dewdrops. The V-shaped leaf appears to an insect to a be cushy pad within from which to suck the nectar. 6 hairs sit on the pad; no noteworthy inconvenience to the diner.

A fly lands, or spider/ant/beetle/grasshopper crawls in and starts to enjoy a meal. In its wandering about the leaf, the insect inadvertently trips a single hair-trigger.

Nothing seems to happen. But the plant takes note, setting a timer of 20 seconds – a test to see whether the trigger was some stray bit flying by, as opposed to a roving rube about to become a feast.

If the intruding insect traipses over another hair within the 20 seconds, or toggles the same hair repeatedly, the meal ticket is made. The 2nd hair zap snaps the jaw leaves shut in a 1/10th of a second.

When the trap is triggered, cells on the green outer surfaces of the leaves expand while the inner pink surfaces don’t. Pressure rises as the outer surface pushes inward. The convex leaves flip to a concave shape, slamming the trap shut in a process known as snap-buckling.

The flytrap is a monarch of thigmonastic movement. The trap snapping entails a multifaceted interaction between tissue turgor and elasticity, triggered by ionic action.

Like many plant movements, the Venus flytrap hair-trigger causes an electrical action potential that induces ionizing calcium channels. In this instance, sufficient ionization to trigger the trap requires 2 stimulations. If only 1 hair is triggered, the ion-channel-based short-term memory dissipates.

As a victim attempts to escape, it touches trigger hairs repeatedly. The flytrap keep count as a way to estimate prey size, and thereby know how much digestive juice is needed.

The flytrap digests its meal over 10 days or so, then opens its jaws for the next victim. All that is left of the last meal is a chitin husk: the exoskeleton of the last diner cum dinner.

Being trigger-happy does not mean being a sap. If some indigestible debris triggers closure, the plant realizes its mistake and rapidly opens again. Further, the secretion of digestive enzymes is not initiated.

Despite its name, a flytrap hardly eats flying insects, which make up only 5% of its intake. Ants = 33%, spiders = 30%, beetles & grasshoppers = 10% each.

Flying insects are, however, highly valued by the flytrap. Come time to propagate by pollination, a plant hoists a flowering stem high above the killing fields of flytraps close to the ground. Flies, bees, and wasps are welcome to taste some nectar and ferry pollen between flytrap plants.

Flytraps take 4 to 5 years to reach maturity from seed and can live 20 to 30 years.

This quick-draw tale comes with a reminder that the flytrap has no muscles, nor recognizable nervous system, nor identifiable intelligence system. Like its intent, the flytrap keeps its wiles well tucked away from inquisitive eyes and hungry flies.

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Nastic movements are provoked by environmental stimuli but are reasoned responses: independent of the direction of the encountered stimuli. Nastic movements are accomplished by changes in water pressure in certain plant cells.

 Shooting Seeds

Nastic movement was an early adaptation, and it has numerous variations.

Spore-producing plants, including mosses, liverworts, and ferns, propel their spores into the air via mechanical motion triggered by moisture loss. This spreads spores away from the parent plant.

Several flowering plants resort to explosive seed distribution. The conventional mode is a seedpod that is sprung against itself so that, as it dries, it gains torque stress: ready to burst at the slightest touch. The marsh geranium spreads seeds this way, as does the touch-me-not.

The squirting cucumber goes one better. Pressure builds within as its fruits mature, until an entire fruit shoots away, with seeds blasting out from the stalk end at high speed. A squirting cucumber may propel its seeds up to 13 meters.

As the squirting cucumber has self-contained seed dispersal, it most certainly does not want its fruit eaten. To that end the fruit is poisonous: loaded with a bitter, steroid-based poison: cucurbitacin.

Many flowers employ a spring-loaded mechanism to launch seeds or dust a pollinator. The flowers of legumes, such as peas, beans, and alfalfa, hold their stamens between paired petals which form a keel at the base of the flower.

The opening trigger requires just the right heft. Nothing happens if a tiny fly alights. But if a bee lands, the petals burst open. The stamens shoot upwards like an uncoiled spring, dusting the insect with pollen.

This is unpleasant to pollinators. Many bees learn to avoid the experience by alighting on the side of the flower and sipping nectar by cautiously reaching between petals.

Most mistletoe species produce sticky seeds which often adhere to birds, who scrape the seeds off when grooming. The dwarf mistletoe takes a different approach. The fruit of a dwarf mistletoe swells as the seeds within mature, until the fruit bursts, squirting seeds at 100 km/hr; fast enough to go far enough to reach a neighboring tree, thus propagating the parasite.

Even some seeds respond to humidity to get where they need to go. The storksbill plant launches seeds using a spring mechanism as its fruits dry. This is but the start of moisture-based movement. Once a seed is on the ground, its spiral awns (slender bristles) coil and uncoil as humidity changes. This propels a seed across the ground until it encounters a crevice. Then its twisting movement screws the seed into the ground. A storksbill seed manages its own self-burial.

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Nyctinastic movements are responsive to the diurnal cycle (daily day & night cycle). Prayer plant leaves lay back during the day, then fold leaves upright for evening prayers. Legumes also show such movement.

Plants with leaves that take a vertical orientation in the dark are nyctinastic. Pulvinus cells, which afford growth-independent movement, accomplish the feat.

Often, the time scale of plant movement is slower than human appreciation permits, but not always. Thigmonastic movements are responsive to touching or shaking a plant. Thigmonastic movements can be quite sudden.

 Pollinator Responder

Endemic to the Peruvian Andes, Nasa poissoniana produces a star-shaped flower. Based upon pollinator activity, these plants gymnastically wave their stamens to maximize pollen distribution.

Individual plants adjust the timing of their pollen presentation to the actual pollination scenario they experience. ~ German botanist Tilo Henning et al

Circumnutation

Proprioception is more than static spatial orientation, just as plant movement is more than reactive tropism. Plants dance in numerous ways for various reasons, including determining which way to go.

Growing shoots move in a recurring spiral oscillation, which Darwin termed circumnutation. The range and speed of circumnutation varies by species, and by both internal and external circumstances.

Strawberry branches move mere millimeters, while bean shoots have wild swings: up to 10 cm in radius.

Vines are vigorous circumnutators. Tendrils search for something to grab onto, sensing various inputs to guide it.

For climbing plants, winding around a potential support is at first tentative. If a plant deems a support unsuitable it unwinds and grows elsewhere. Tendrils reject something too smooth, as its long-term prospects are problematic.

Tendrils of the same plant recognize one another and will not coil about each other. Tendrils are also reluctant to attach themselves to those of a different species.

Speed is another aspect of circumnutation. Tulips steadily swirl at a fixed speed, taking 4 hours to make a circuit. Wheat completes a rotation every 2 hours.

Other species are more various. Cress stems take anywhere from 15 minutes to 24 hours for a single circle.

 Dodgy Dodders

A 5-angled dodder swirls its shoots about, reaching with orange tentacles to grasp the slim stems of nearby tomato plants and suck them for nourishment.

While dodders have roots that provide some nutrients, they don’t bother with photosynthesis. If a young dodder can’t find a plant to parasitize, it doesn’t have long to live.

A dodder is careful in its search. It won’t touch a leaf so as not to tip off its prey. It instead sinks down to grab the stem.

A dodder knows where to go by scent. It sniffs its way to a meal, then makes its move.

A dodder is selective: picking the most nutritious prey. Given a choice of wheat or tomato, a dodder will put the squeeze on marinara, passing by pasta.

The dodginess is just getting started once a dodder has its grips on a host plant. Dodders directly manipulate their victims genetically to keep nutrients flowing to them – something the hosts would dearly like to thwart.

A dodder passes microRNAs into its host plant that regulate the expression of host genes in a very direct way. The microRNAs specifically target host genes that are involved in the plant’s defense against the parasite. ~ American plant pathologist James Westwood

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Quick movements by plants are rare, but plants as universally moving beings is a truism. The contrast to animal movement is only one of timescale, in that plants are typically plodding in their physical behaviors. Ultimately it is the pace of living that is relative.

Growth as Movement

The ability to generate new roots confers plants a high degree of developmental plasticity. ~ German botanists Ricardo Giehl & Nicolaus von Wirén

Behavior in animals often corresponds to movement. Plants move by changing form: phenotypic plasticity. Growth and other changes in roots, stems, leaves, and flowers are all chosen behaviors which involve movement, though not how animals move. Plants forage strategically and fight each other for resources in what we would regard as extreme slow motion.

Most higher plants are modular in structure: the plant body is plastic, molded purposefully, with variable numbers of roots, branches, leaves, and buds. This plasticity enables a plant to alter its phenotype, accurately occupying local space based upon existing conditions.

 Roots on the Move

Roots are the primary provider of water and minerals in most higher plants. In return, roots receive sugars from the shoots, which are employed for building more root tissue.

Roots being able to drill their way through stiff soil is essential. The strength of root fibers is indeed impressive when considering that hard soil is squashed as a root grows. Soil compaction is how roots help prevent soil erosion – an integrity essential to a plant’s livelihood.

There are 2 basic root types: taproots and fibrous root systems. Each type is suited for specific conditions. Root types are not mutually exclusive.

Taproots are an adaptation to getting at water deep in the soil. Many desert plants have taproots.

Taproots often store water as a reserve. Taproot plants send out secondary roots to prospect for opportunities.

Fibrous root systems spread near the surface, where most plant minerals are located. Fibrous roots are the product of careful consideration.

Making the right choice about where to deploy new roots can determine survival, especially when soil resources are scarce and unevenly distributed. Plant roots respond to gradients of soil moisture by favoring the formation of lateral roots toward sites with available water. ~ Ricardo Giehl & Nicolaus von Wirén

Plants hunt by sending out roots, growing in promising patches and withering in dead zones. Root systems not only sense the soil volume where they grow, they also recognize and discriminate against the roots of others, even their own kind.

As part of self-recognition, roots of any individual plant spread in a way to occupy as much soil space as possible while avoiding contact with siblings. The original pioneers of real estate, plants are territorial.

Darwin experimentally showed how seedling roots sensed signals of touch, light, moisture, and gravity, all simultaneously. Using sensory integration, growing roots discriminate which cues are the most crucial. Touch and humidity may override gravity, and so a root may travel sideways or even up.

Natural soil is heterogeneous, with a wide spectrum of both texture and distribution of resources on offer. To thrive, a plant must correctly assess outcome probabilities and from that construct roots’ shapes and growth direction.

Plants adapt to heterogeneous soil conditions by altering their root architecture. ~ Indian-British botanist Ari Sadanandom et al

Competition

The survival and growth of an individual plant may be strongly influenced by competition with its neighbors. ~ American ecologist David Tilman

Plants are well aware of their situation. They constantly assess their prospects and grow in a decided direction.

If plants were unable to discriminate between unoccupied soil and those containing competitors, they would soon be eliminated by fitter individuals. ~ Anthony Trewavas

Because they sense their neighbors, trees do not crowd each other. Growing trees avoid already-shaded areas and shape their growth so that their leaves have their time in the Sun.

Branch shoots sprout a few leaves, getting feedback about local conditions. With a poor response the plant withers the branch.

Similarly, as trees grow up, in a race with neighbors to sunny heights, they wither the lower branches that no longer fetch enough light energy.

Flowers, shrubs, and trees all change their phenotype intelligently: foraging for resources, competitively excluding potential rivals, and constructing as fulsomely as possible.

Competition for resources is a way of life for wild plants as much as it is for all animals.

Plants are limited in their ability to choose their neighbours, but they are able to orchestrate a wide spectrum of rational competitive behaviours that increase their prospects to prevail under various ecological settings. Through the perception of neighbours, plants are able to anticipate probable competitive interactions and modify their competitive behaviours to maximize their long-term gains.

Specifically, plants can minimize competitive encounters by avoiding their neighbours; maximize their competitive effects by aggressively confronting their neighbours; or tolerate the competitive effects of their neighbours. The adaptive values of these options are expected to depend strongly on the plants’ evolutionary background, and to change dynamically according to their past development, and relative sizes and vigour. ~ Israeli evolutionary botanist Ariel Novoplansky

Soybeans were tested in soil boxes with removable partitions. With the partition in place, each plant had its own soil and experienced no competition. These were soil owners.

Remove the partition and each plant has double the space available but faced competition in having to share soil with another plant. These were soil sharers.

In the absence of competition, the soybeans constrained their roots. But when sharing soil, competition drove the plants to increase root production. They even confrontationally turned their roots in the direction of the rival.

Competition has its costs. Owners had a much higher shoot-root ratio, and nearly double the seed yield of sharers. But plants cannot bear the prospect of handing over soil resources that may be forgone at a later date.

The canopy of every forest signifies a fight for light. That competition begins underground.

Spotted knapweed, native to Eurasia, caught a ride to North America. An inveterate land grabber, knapweed blankets entire slopes, pushing out native vegetation. Its weapon: catechin, a phenol which can retard plant growth. Having grown used to knapweed’s ways, European plants neighboring knapweed are not bothered by its catechin seeps, but some North American plant species have not adapted, and so are overwhelmed. But not all. Lupin and blanketflower fight back by exuding extra oxalate: 4 times the normal level for blanketflower, and up to 40 times normal for lupine. Oxalate neutralizes catechin and extends a blanket of protection not only to the defender, but also neighbors.

Many plants employ secondary metabolites to establish territories for themselves and their offspring. Various phytochemicals, including oils, alkaloids, steroids, terpenes, and coumarin derivatives, are poisonous to botanical rivals.

Black walnut and eucalyptus trees are exemplary of fierce allelopathic warriors via root secretions. Allelopathy is the production of biochemicals intended to affect the health of other organisms.

Crabgrass does not just crowd out other plants. It kills them with herbicides released from its roots.

California purple sage pops terpene into the air, where it disperses and is then absorbed in the soil. This promotes clearing zones around sage shrubs.

Trees, particularly conifers, actively release terpenes in warm weather. The terpenes act as a natural cloud seeding, encouraging cloud formation. Trees regulate forest temperature by creating clouds that block sunlight.

Heath allelopathically inhibits pine trees by chemically inhibiting the soil fungi that share a symbiotic relationship with pine roots. The Scottish harvested the pine forests in Scotland at the advent of the Industrial Revolution as fuel for industry. The land is now colonized by heath, which thwart pine reforestation efforts by foiling the fungi needed for pines to make a comeback.

Belowground

Plant roots are exposed to an enormous amount of soil biodiversity; a handful of soil can contain more than 5000 species that operate together in plant-soil feedback. ~ Wim van der Putten

Plant roots are a magnet to soil bacteria and fungi, whose concentration in the rhizosphere is 100 times that of surrounding soil.

Roots recognize microbes that are helpful and those that are trouble. Beneficial bacteria are essential for robust roots. Bacteria nest in roots and then protect their homestead.

In the rhizosphere, the plant-associated microbiome is intricately involved in plant health and serves as a reservoir of additional genes that plants can access when needed. ~ American botanists Marnie Rout & Darlene Southwort

The rhizosphere is awash with metabolites exuded by plant roots. The choice of metabolite at a root meristem depends on whether friend or foe is at the receiving end.

Roots invite beneficial microbes by sugars and organic compounds that are energy rich. Those unwelcome are treated to hostile chemical concoctions, in attempts by roots to fortify themselves against intrusion. Those same pest toxins recruit specific species which have evolved tolerance.

Benzoxazinoids (BXs) are a class of secondary metabolites used against pests aboveground and below. Roots put out BXs early in life, when they are most vulnerable. The beneficial rhizobacterium Pseudomonas putida is not put off by BXs. It treats BXs as a beacon to find suitable employment.

Plants put out a welcome mat to helpful microbes, providing the molecular building blocks to promote beneficial colonization of roots. Plants dispense specific sugars from their cell walls that activate bacterial genes which induce biofilms. The rhizosphere becomes a rich nesting site for a microbial community beneficial to a plant.

Early in its growth cycle, a plant is putting out a lot of sugars which many microbes like. As the plant matures, it releases a more diverse mixture of metabolites. The microbes that become more abundant in the rhizosphere are those that can use these metabolites. ~ American microbiologist Trent Northen

Most plant species nip their nitrogen from the soil themselves. But many plant groups get their supply via cooperative and symbiotic relationships with nitrogen-fixing bacteria and fungi.

Soil bacteria and fungi reduce atmospheric nitrogen (N2) to ammonia (NH3), but they can only fix nitrogen when intimate with a plant by being welcomed. Plants capable of such symbiotic relationships grow better in nitrogen-poor soil than do competitors without an ally for nitrogen fixation.

The soil is not the only place that nitrogen-fixers reside. Bacteria that live in leaves also fix nitrogen.

The same stresses that affect plants affect the microbes that live among them. For the sake of all concerned, microbes help plants adapt more quickly than they could otherwise.

Drought stress affects microbes, and they, in turn, drive plants to flower earlier and help plants grow and reproduce when faced with drought. ~ American botanist Jennifer Lau

Cross-species mutualisms are ubiquitous among all life. Many have a lopsided power balance, where one partner, often a plant, can even kill a misbehaving helper. Others are more mercenary.

Endophytes

Soil conditions are seldom ideal. When the situation in the soil shifts from sanguine to stressful, plants make the logical move: they ask for help.

Plants under abiotic duress enhance their ability to deal with the dilemma by recruiting endophytes to assist with the necessities of living. In return, plants offer the comforts of home to those microbes they take in. An endophyte is a plant endosymbiont – most commonly a bacterium or fungus – in a mutual or commensal relationship.

95% of plants get fed with help from friendly molds. Fungi attach to plant roots and then shoot out fibrous filaments, infiltrating bacterial corpses. Then fungi feast on the succulent cellular matter. Spillover provides plants with nutrients such as nitrogen and phosphorus.

The bacteria that live in stems and branches provide nutrients to their host plants, including nitrogen fixation, in return for a high-rise home and free meals.

Not only do endophytes sup with their vegetative friends, they often provide protection. Extensive colonization of plant tissue by endophytes creates a barrier against invasion. Endophytes outcompete their rivals, preventing pathogenic organisms from taking hold by producing chemicals which inhibit the growth of the competition.

Fungi actively shield plants from various diseases, bacteria, insects, and roundworms. They can also promote production of alkaloids that deter herbivores.

Some endophytes are host-specific, but many colonize numerous plant species. Likewise, a single plant sports many endophyte species, both fungal and bacteria.

A plant’s microbiome can even vary among different plant parts. One branch on a plant may have a substantially different microbial community than another.

 Mycorrhizae

A mycorrhiza is a mycelial fungus that has a symbiotic relationship with a plant. The term is often used to denote the association as well as the fungus. This symbiosis has been going on for at least 470 million years.

The relationship typically begins with plant roots advertising for fungal services by releasing the hormone strigolactone. Plants in phosphate-deficient soils are especially desperate, and so vigorously solicit.

Mycorrhizal mutualism works in several ways; 2 forms are endomycorrhiza and ectomycorrhiza.

In endomycorrhiza, a fungus colonizes the host plant’s root cells, often after considerable negotiation which results in the plant letting the fungus past its defenses. To further their accommodation, fungi may request more lateral root formation, which the plant obliges. Such development is also a common response to roots in search of phosphate.

Nutrient exchange commences. The intricacy and intimacy of this fungal symbiosis is extensive.

4–20% of the carbon compounds that a plant produces from photosynthesis flow to the fungus, as does considerable quantities of sugar, lipids, and some vitamins. In return, mycorrhizae furnish minerals and other foodstuffs to the plant.

Phosphate is the macronutrient hardest for plant roots to come by, partly because it is readily absorbed into soil particles. Endomycorrhizal symbiosis enhances a root’s phosphate uptake 2 to 6 times from that of uncolonized roots.

Though the symbiosis is long-standing, the fungus and plants have never merged. They are still separated by cell membranes.

To enable exchange of the relatively large sugar and phosphate molecules, plant cells construct protein complex conduit tunnels; a task backed by knowledge covered in over 800 genes.

Plant roots can tell which fungal threads are providing an abundance and reward them. Likewise, the fungi know, and preferentially reward, a succulent supplier, while shunning slackers.

This marketplace mechanism functions because either plants or their fungal symbionts can switch partners. Mutual liberty allows cooperation enforcement by both parties. Such efficient exchange may explain why mutualism never evolved into merger.

Over 80% of land plants have endomycorrhizal relations, including grasses, most crop plants, many shrubs, trees, and flowers.

The quality of relations between fungus and plant varies considerably. Some plants have very productive friendships with their fungi, while others have not sufficiently coevolved for mutual benefits to have blossomed.

In ectomycorrhiza, the fungus enters the root, but not the root cells. Ectomycorrhizal plant hosts include pines, firs, spruce, and oaks; mostly forest trees, as well as several other plant species; totaling 10% of plant species. These fungi often form mushrooms or truffles. Working for plant roots is just their day job.

Laccaria bicolor shows itself as a small tan mushroom with lilac gills. Laccaria sets up shop at the roots of temperate trees, notably the eastern white pine.

Once nestled in, the fungus chemically lures springtails, which the fungus then kills with a toxin. Laccaria dines on its kill by sucking up nitrogen from the dead insect using its filaments. The fungus passes a goodly portion of the nitrogen to the pine.

This arrangement provides ~25% of the white pine’s nitrogen. In a show of gratitude, the plant showers the fungus with energy-rich carbohydrates.

Some fungi have mycorrhizal relations with multiple plants at the same time. A group of plants interlinked through a common mycorrhizal network is termed a guild.

Nutritional exchange is networked between plants and mycorrhiza mycelia. This trade can be substantial indeed.

 Trees

The forest is more than the sum of its trees. ~ Swiss botanist Christian Körner

Forests are a hotbed of competition, as saplings struggle for enough light to make their way to the canopy. This belies a congeniality that goes on belowground, especially among mature trees.

Up to 40% of the carbon in the fine roots of one individual may be derived from photosynthetic products of a neighbor. ~ Swiss botanist Tamir Klein et al

Mycorrhizae have an interest in keeping the guild above them healthy, so they allocate their surplus accordingly.

That 4% of net primary productivity is transferred to neighboring trees suggests that carbon is a nonlimiting resource, and not growth-limiting for large trees. Thus, carbon allocation and loss to mycorrhizal fungi does not necessarily impair plant fitness. The exchange of “nonlimiting” carbon for nutrients may be one of the key factors responsible for the evolutionary stability of the mycorrhizal symbiosis. ~ Dutch evolutionary biologist Marcel van der Heijden

 Thale Cress Pickiness

It was long thought that the sole role of the immune system was to distinguish friend from foe and vanquish the unwanted. An immune system is instead more a microbiome management system.

While most plants employ a mycorrhizal mesh to get the phosphate and other minerals they need, thale cress does not. Instead, the plant selectively allows the fungus Colletotrichum tofieldiae around its roots only when it needs help mining minerals from the soil. If phosphate is plentiful, the fungus is rejected by the plant’s immune system.

The thale cress plant controls its interaction with its tenant by linking its immune system to a sensor for phosphate availability. It’s a fantastically well-regulated system. A foe is recognized as such only in specific circumstances. ~ German botanist Paul Schulze-Lefert

The beneficial interaction between Colletotrichum and thale cress is surprising, in that this fungal family is almost everywhere a plant pathogen. But the central plateau of Spain, where this selective mutualism occurs, is a difficult environment, with scant soluble phosphate in the soil. Thale cress managed to strike a deal because otherwise it could not survive, and the fungus would go wanting.

The mutual coexistence is beneficial to both partners, but only as long as the right conditions prevail. ~ Paul Schulze-Lefert

 Trichoderma

Trichoderma are a filamentous fungus with a diversity of lifestyles. But all are sociable with other fungi, plants, and animals.

Trichoderma are ubiquitous opportunists: they are everywhere in the soil, and the life they choose depends on external conditions. These mold fungi can grow inside of plants, on roots, on the bark of a tree, or even on top of other fungi.

Trichoderma have a sweet tooth. Sugar is their steady diet. But trichoderma are not sweeties. They carry an arsenal of chemical weaponry that comes DNA-encoded.

To serve their self-interest, Trichoderma can attack other fungi or bacteria with powerful toxins. For this reason – their ability to antagonize plant pathogens and promote plant growth – Trichoderma are valued plant symbionts.

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 Mycoheterotrophy

Not all plants act mutualistically with mycorrhizas. Some practice mycoheterotrophy: robbing neighboring plants of nutrition by tapping into the mycorrhizal network which acts on behalf of a plant guild.

 Indian Pipe

Indian pipe is a plant indigenous to the temperate regions of North America and eastern Asia, though its appearances there are scarce.

The Indian pipe is also called the corpse plant and ghost plant, as it is a stark white. This is because this peculiar vascular plant does not have any chlorophyll.

Given its looks, the Indian pipe is easily mistaken for a fungus. It grows up through dead leaves on the forest floor, to a height of 15–20 cm; strikingly like a fungal fruiting body.

The mycorrhiza that work with a nearby tree share some of their spoils with the Indian pipe, transferring nutrients to the pipe’s roots. There is no direct connection between the Indian pipe and the tree.

What the mycorrhizal fungus gets out of a relationship with the Indian pipe is not known; maybe just the leftovers of the corpse plant’s corpse when it dies.

Orchids

All orchids, at some stage in their life cycle, lack photosynthesis. Some lost their photosynthetic ability altogether. Orchid mycorrhizae are critically important during orchid germination, as orchid seeds have no energy reserve. So, all orchids, either sometimes or all the time, are mycoheterotrophic.

Sociality

Plants sense whom they are interacting with. ~ Canadian evolutionary plant ecologist Susan Dudley

Via root interactions, plants detect if they are in the presence of a close relative and grow differently if in friendly terrain. Kin recognition and corresponding comity has been seen in cabbage, heywood, jewelweed, mustard, rice, sagebrush, searockets, and sunflowers. Only recently have botanists turned their attention to this area of study. Doubtlessly sophisticated sociality is ubiquitous among plants.

 The Comity of Sunflowers

Sunflowers track the Sun, facing east before dawn and ending the day facing west. Growing close together in a field, sunflowers also keep track of each other. Sensing that their sunlight is less than optimal, a sunflower leans away from another one by about 10° to get a little better light. This creates a coordinated cascade effect among neighbors, who lean in opposite directions to get better light, but also try not to interfere too much with their neighbors.

Sunflowers also care about their bee pollinators: packing a poison into their pollen that wards off brood-parasitic wasps which otherwise afflict the bees.

 Jewelweed Relations

Jewelweed is a flowering plant that lives in the forest understory, where the soil is nutrient-rich, but an abundance of light is hard to come by. With hearty soil below, the competition is aboveground for light access. Plants that gain the upper leaf not only enhance their growth potential but also put others nearby in the shade, limiting the growth of rivals.

Jewelweed aggressively competes with its neighbors: extending stems and leafing as fast as it can. But having a relative for a neighbor changes a jewelweed’s growth strategy: accommodating its kin by sharing light access. It balances its tempered leafing with more abundant root growth.

 Searocket Siblings

There is little competition for sunlight on a sand dune near the seashore. The battle is below, in the sandy soil, for enough nutrients.

Searockets are a flowering annual with fleshy leaves; a member of the mustard family. A searocket rises from a long taproot.

Searockets in the presence of strangers vigorously grab all the nutrients that they can. But a searocket among family is more subdued, giving its siblings a fair share of the spoils in the soil.

All Together Now

The reason trees share food and communicate is that they need each other. It takes a forest to create a microclimate suitable for tree growth and sustenance. It’s not surprising that isolated trees have far shorter lives than those living connected together in forests. ~ Australian environmentalist Tim Flannery

Comity is not confined to kin, especially when the community is at risk. Plants inherently understand the interrelated nature of their existence as a commonwealth. We only now seem to be learning what plants have long known: that biodiversity is essential to ecosystem health.

In numbers, there is strength. Buckhorn is a weed common on cultivated land in the British Isles. Buckthorn is more resistant to a fungal pathogen, and thus less likely to be colonized by it when the plant is well connected with its siblings in the local population.

The buckhorn example highlights that plants are more vulnerable when their habitat is fragmented. As plants face stresses such as drought, they treat their neighbors kindlier, promoting survival for all rather than competing.

These findings were consistent across fitness measures, stress types, growth forms, life histories, origins, climatic zones, ecosystems, and methodologies. ~ Chinese botanist Qiang He

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With predators constantly on the prowl, it is a dangerous world, especially when one is rooted to a spot. Plants rely upon their wits and astounding alchemy to survive.

Plants & Animals

Animals are something invented by plants to move seeds around. ~ American ethnobotanist Terence McKenna

In offering treats as recompense for pollination and seed dispersal, plants have done their best to put up with animals. While some animals have been bought off, their appetites seem insatiable. Hence, plants evolved numerous ways of eliminating unwary predators. Poison is popular, as are prickly parts to deter attempts at consumption.

One of the great mysteries of evolutionary biology is how plants know exactly what it takes to disrupt the development cycle of a common pest or deliver a nasty surprise to the nervous system of some plodding herbivore. The answer is that Nature furtively exercises its own intelligence, only manifest in local effect.

Some plants turn the tables and consume the critters that otherwise would be nuisance. The actual numbers of carnivorous plants are much greater than the few species found in textbooks.

Despite decent defenses, being eaten alive by craven beasts is all-too-often unavoidable. Plants rely upon their modular development and innate sense of achievement to let bygones be and get on with growing their way past losses.

The regenerative powers of plants are wondrous. About 1,000 species can be reborn from a mere residual of root, and a few hundred can come back from a scrap of leaf.

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Pollination seems a sanguine event: bees industriously buzzing about flowers to collect the ingredients to make their delectable honey, while inadvertently helping the plants they favor in return. But pollination can have a sinister edge: a product of deception and incarceration. For pollination as a scam, nothing beats the elephant yam.

 Elephant Yam

The jungle cools down at night, but in a few spots it is getting very warm: 10 °C above ambient. A witch is brewing her potion.

The Chinese call the elephant yam “the witch of the forest.” She has an imposing presence: a girth of up to 50 cm, and up to 40 cm high. Once pollinated, her triumph takes her to 2 meters.

In a cloud of pungency, the witch releases the noxious stench of rotting flesh. The stink annoys some, but nearby carrion beetles are thrilled. In the dark of night, they come as quick as they can.

A beetle clambers upon the single large leaf. On the inside, the leaf is slippery. The beetle tumbles in, to the base of the flower.

The beetle naturally tries to scale the leaf to escape but is unable. The slippery waxy wall cannot be surmounted. The beetle is stuck.

Night turns to day. Captive beetles remain restless but powerless.

As twilight dims again, the witch showers her subjects with sticky pollen from the stamen that surrounds the flower. As the beetles once again try to leave, they find that the walls holding them in have changed texture, and now can be scaled, allowing them to leave.

Burdened with pollen, and in need of a good meal, the beetles smell a feast not too far away. Another witch beckons.

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Many plant deceptions merely aim at competitive advantage. To attract more traffic, some orchids mimic the female mating signals of their pollinators.

Besides obfuscating orchids, there are deceitful daisies in South Africa that mimic female insects. Naïve pollinating flies fully fall for the lure the first time around, but soon learn that a daisy does not live up to her scent.

Other orchids mimic signals of prey, to rope in insects that otherwise would forgo pollination duty.

Though it needs help moving pollen, a lady-slipper orchid in southwestern China lays out no nectar or other food to attract a pollinator. What it offers are smelly, black, hairy spots that mimic mold. Fungus-seeking flies flock to it, and so are deceived into porting pollen by being fooled time and again.

 Survival by Theft

Seed dispersal is essential to many plants and trees for population survival. Fruit is a common incentive to facilitate animals spreading seeds.

The Neotropics (tropical Central and South America) are rich in woody species that bear large-seed fruit. 10,000 ya, megafauna, such as mastodons and gomphotheres, effectively scattered seeds by defecating far from the source of their meals. The extinction of these megafauna by human hunting carried the risk that these tree species would also go extinct. Yet several survived.

Agouti are a rodent endemic to the Neotropics, related to guinea pigs, but more rat-like. Macaws and agouti are the only species known that can open Brazil nuts without using tools; a feat that takes strength and exceptionally sharp teeth.

Like squirrels, agouti hoard food in numerous small, buried stores. Like rats, agouti are wily thieves, readily raiding caches that others bury. By repeated theft and recaching, a seed may be moved many times; sometimes 30 or more.

Not all cached seeds are eaten. 14% survive to germinate the next year.

Germination is no guarantee of a good life. It is in a plant’s interest of legacy that its seeds are not dispersed to a location where seedlings will face competition from their own kind (conspecifics).

An agouti tends to move its seeds away from conspecific trees, as it figures a more remote location helps avoid pilfering by other agoutis. Such strategic thinking benefits an astute agouti and the seeds it caches.

 Just Don’t Munch the Seeds

Taily weed plays an especial role in the desert ecosystem it inhabits. The weed sometimes serves a nursing role in helping other plant species become established. For animals, the taily weed produces thousands of tiny berries year-round.

Many birds, rodents, ibex, and camels eat the plant or its berries. Among them is the Cairo spiny mouse, which ekes out a living in the rocky hills and hot deserts of north Africa; eating seeds, desert plants, insects, and snails.

The spiny mouse eats taily weed berries, but carefully. Each berry has 5–9 seeds. If a seed is chewed, enzymes within activate a toxic glucosinate in the pulp, which is otherwise harmless. The tiny spiny mouse cannot afford to pay that price.

To avoid detonating this pungent mustard bomb, the spiny mouse spits the seeds out before consuming the berry. Because of the taily weed’s chemical cunning, the small-seed eater is turned into a good seed-disperser. Other small rodents that dine on the taily weed take the same precaution.

Because birds swallow their food whole, taily weed seeds pass through without setting off the mustard bomb.

Taily weed is not the only plant with this ploy. Mammals don’t eat chili peppers because they can’t take the heat from the capsaicin that the seeds contain. Birds who do eat the fruit don’t crush seeds when eating, so the heat of the seed is forgone.

 Cliffhanger

Ants are frequent floral visitors but are often dismissed by biologists as lackluster pollinators owing to their small size, limited foraging range, and antibiotic metapleural glands, which may reduce pollen viability. That assessment underestimates their worth as plant pals.

Ants are often as valuable as more highly regarded winged insects. Ant pollination is especially important in regions adverse to flying insects, such as mountains and deserts.

Numerous ants, such as harvester ants, are seed predators. But ants also play a vital role in seed dispersal.

For most plants, the pollinator and seed-disperser differ. In habitats poor in animal diversity, plants take advantage of whatever resources are available. Several island plants employ lizards, birds, or flying foxes as double mutualists.

Mountain cliffs are ecological islands; one of the most resource-poor places a plant can be. It takes ingenuity to hang tough. The upside to a cliff comes with protection against climatic extremes and large herbivores.

Rock plants are typically small and long-lived, with stable populations. For those species that do adapt, mortality is low.

The serious challenges for cliff dwellers remain pollination and seed dispersal. The small, slow-growing Spanish Pyrenees plant Borderea chouardii, which can live over 300 years, has enlisted 3 ant species as assistants.

chouardii are either male or female, but not both. They do not self-pollinate. 2 different ants act as pollinators, while a 3rd species disperses seeds.

Relying upon ants is a risky strategy. But B. chouardii, which has doubled down on its mutualism, has keep the band together for a long time: having been around from just after the dinosaurs died off, back when the Pyrenees was tropical.

 Myrmecochory

Myrmecochory is seed dispersal by ants. ~23,000 plant species bribe ants to take their seeds by coating them with elaiosomes: fleshy seed caps rich in lipids and proteins. Ants cart such seeds to their nest and serve the elaiosome to their larvae.

Plants compete for ant services. Ants prefer larger seeds to smaller ones, so small-seed plants release their seeds in early spring, when ant foraging is unreliable. This advance release avoids competition from large seeds, which are let go later, when ant populations are at their peak.

The fatty acid content, particularly oleic acid, is the main trigger to prompt ant seed pickup.

Plants know what nourishes ant larvae. Some target their elaiosome content to entice specific ants, thus enhancing their fitness by choosing the most effective dispersers.

Some plants cheat, by coating their seeds with enough temptation to be taken, but not provide the ants with nutritious food. Puschkinia and Hepatica are exemplary myrmecochory cheaters.

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Plants literally take care of animals, providing the air they breathe and the food they eat. Plants provide the foundation ecology for most animals, whether aquatic or living on land. Carnivores would not exist without herbivores.

Plant pathogens and pests – from microbes to the vast mass of the animal kingdom – are a continuous threat. Some have been co-opted into providing nutrients or pollination, but the odds always run against the vegetation.

To ward off what would otherwise be a slaughter, plants have defenses besides robustness and strategic chemical cunning. Bark and spikes (thorns, spines) provide some protection against penetration. Sometimes it is enough to hide in plain sight.

Camouflage

Hiding from herbivores is not easy, but a few plants manage the trick via camouflage.

 Stone Plants

Stone plants are succulents, native to the dry lands of southern Africa. Succulents are adapted to arid conditions by their ability to internally retain a reserve water supply.

Stone plants are aptly colored: dull creams, grays, and tans; sometimes with speckles, or lines that resemble sedimentary rocks. The shape of the leaves complete the effect.

A stone plant has 1 or more pairs of bulbous leaves, opposite each other, almost fused together, shaped like small rocks. The leaves are mostly buried. A leaf’s top surface is translucent, to let light into the interior for photosynthesis. (The efficacy of stone plant camouflage is eerie; an impressive example of the coherent intelligence behind the diversity in Nature.)

During winter, a new leaf pair is prepared, growing inside an existing fused pair. In spring, the new pair, like pebbles rising, appear between the old leaves, which then dry up.

In autumn, a leaf pair produces a white or yellow flower for pollination, typically via flying insects. Stone plants are obligate outcrossers which require cross-pollination with another plant. Outcrossing – the introduction of unrelated genes – promotes genetic diversity.

During drought, stone plant leaves shrink and disappear from the surface. A plant minimizes itself belowground.

 Pygmy Pipes

The pygmy pipe is a member of the blueberry family. Pygmy pipes are native to the Appalachian Mountains in the southeastern US but are not prolific in their home range; indeed, they are an uncommon sight.

Pygmy pipe flowers bud in a light lavender that deepens within a few weeks. The flowers come covered by bracts (specialized leaves).

The bracts and the flower’s sepal (flower support wrapping) are a toasty color that looks like leaf litter. The effect is enhanced by the bracts and sepal quickly drying out, giving the fragrant flower a decided well-past-its-prime look to herbivores, while retaining the scented appeal that pollinators appreciate.

Deception is not the pygmy pipes’ only wile. Pygmy pipes are thieves by livelihood.

Pygmy pipes do not contain chlorophyll. Instead, they get their nutrition by robbing the local mycorrhizal network: mycoheterotrophytes.

Borrowed Protection

Besides food, various animals take advantage of plants’ secondary compounds. Humans use them for healing, as do other animals. Other organisms make plants’ protective chemicals their own protection.

Desert locusts accumulate toxins from the desert plants on which they feed, immune to the terpenes intended to thwart them. Instead, the locusts discourage potential predators who have a less hardy constitution.

Willow trees produce the glycoside salicin, the active ingredient in the European folk medicine used for headaches and gout. Organic salicin is the molecular starting point to synthesizing aspirin.

Chrysomelid beetles that feed on willow trees use the tree’s salicin to synthesize salicylaldehyde, which the beetle employs for its own defense.

 Monarchs & Milkweed

Various milkweeds employ cardiac glycosides to ward off herbivores. The larvae of monarch butterflies evolved the ability to eat milkweed with impunity, concentrating the glycoside compounds into their own body.

Adult butterflies use this weapon against birds that prey upon them. Blue jays quickly learn to take monarchs off the menu by noting their characteristic wing pattern.

The interaction between monarchs and milkweed is ultimately to the milkweed’s favor.

In an exclusive mutualism, monarchs lay their eggs on milkweed. The eggs hatch onto an instant meal, but one that might be an early demise.

If a young caterpillar hits a vein, viscid sap pours out, engulfing the newborn. The latex can drown the little crawler, or lock its jaws, making the gushing bite its last.

The tally by the time the caterpillars pupate is that 2/3rds having been killed by the milkweed. The 1/3rd that flutter away do so to an immediate victory meal.

The milkweed flowers with perfect timing: just as the butterflies emerge, so that the butterflies can enjoy some well-deserved nectar while pollinating the milkweed. In all, milkweeds manage the damage from their dependent pollinators.

The ferocity of milkweed’s latex brew is welcome in the floral neighborhood, as its toxic scent wards away pests, such as wireworms, which would otherwise infect nearby plants.

Viceroy butterflies ply on the poisons that monarchs amass by mimicking the monarch’s warning coloration, thus avoiding predation by masquerading as monarchs. This stratagem only works if adult viceroys emerge late in the season, after the birds have learned that monarchs are a digestive monstrosity.

Plants as Power Plants

Plants are the source of organic carbon – that is, are the food for almost all of the nonphotosynthetic organisms on Earth. ~ English botanist Alison Smith

A plant’s demise is a feast for the fungi and other saprovores that recycle plant matter and stored energy back into the ecosystem. Soil bacteria and a host of invertebrates involved in recycling depend upon these organic compounds, which are of a tremendous variety, and are integral to biomes in diverse ways.

Interdependence is not necessarily contemporaneous. Peat and coal – primary energy sources for polluting humans – are highly compressed ancient plant matter. Coal remains the largest energy source for electricity generation. Petroleum and natural gas too are derived from ancient biomass.

As humans deforest and dump their exhausts into the skies in prodigious quantities, the power of plants goes underappreciated. Each year plants absorb 100 billion tonnes of atmospheric carbon, incorporating it into their own tissue.

This clears 8% of all CO2 in the air. The less foliage, the less scrubbing of the atmosphere.

Plants capture about 4% of the sunlight that beams down to Earth, equivalent to 100 terawatts (trillion watts) of power.

As an organic colony, Earth requires a sustained energy force. Extinction events have demonstrated that when the planet’s organic power plant and ecosystem resilience drop below a threshold, the Earth’s ecology becomes unstable – self-organized criticality in action.

Earth’s biotic power plant consists of plants, which also act as a bulwark of planetary stability. Their loss ensures death to animals.

Medicinal Plants

What the eyes perceive in herbs or stones or trees is not yet a remedy; the eyes see only the dross. ~ Swiss German botanist Paracelsus

When plants first colonized land they were likely to be eminently edible. These plants were the great draw for the evolution of land animals.

Self-preservation meant protection. Plants concocted a cornucopia of complex chemical compounds for a vast variety of intents, many having effects beyond the plants themselves, either to the benefit of collaborators or poisoning their ingesters. This launched an evolutionary race of increasing sophistication in both plant protection and animal digestion.

Microbes and insects were plants’ primary targets. Early on, plants and some microbes made peace by mutualism. Flowering plants would do the same by domesticating pollinating insects.

But the majority of intruders into a plant’s life cycle were pests. In due time, plants’ chemical armor would serve higher animals afflicted by the same species that had been nemeses to plants.

While plants used animals for their own purpose, turnabout in time meant that the secondary metabolites intended to fend off predation became a compelling reason for animals to consume plants. Medicinal plant use by humans is a conclusion in terms of animal evolution.

Plants have always been the foundation of human medicine. Before laboratory synthesis developed in the late 1800s, 80% of the substances used to cure diseases were plant derived. Plants still account for some 40% of the drugs taken, and the inspiration for most others.

For well over a century, loggers in the Pacific Northwest cut and burned the slow-growing Pacific yew tree, an understory evergreen growth to the hardwood trees in the old-growth forests that were the target of the industry.

In 1967, taxol, an extract from the yew, was found as an effective treatment for various cancers. Overnight, the Pacific yew became more valuable than the hardwood harvested for homes and other construction.

The healing potential of plants remains relatively unexplored. Of the over 250,000 species of flowering plants, fewer than 5% have been examined for their possible medicinal use. The equatorial tropical forests being ravaged by humans hold the greatest potential for new discoveries.

Understanding the utility of a plant is one thing; employing it a different matter. Of the 28,187 plants known to have healing properties, only 16% are being used.

History

Like the apes before them, the earliest hominids learned which plants were edible, and otherwise helpful or harmful. These hominids relied upon plants for medicine as well as food. So esteemed were medicinal plants that they have been found buried in graves of people who lived 60 thousand years ago.

 Antiquity

Though versed in math and science, the then-contemporaneous Sumerians, Assyrians, and Babylonians attributed diseases to the nefarious influences of supernatural agents. Divining cures were the province of priests and priestesses.

Herbal medicine was practiced and codified. Sumerian clay tablets from 6,000 ya catalog many botanical remedies.

Shénnóng, the Emperor of the Five Grains, reputedly lived some 2,800 ya. Myth has Shénnóng teaching the ancient Chinese agricultural practices and herbal remedies, having tasted hundreds of herbs to test their medical merit.

Shénnóng is credited with the discovery of tea, which allegedly acts as an antidote for the poisonous purports of some 70 herbs. Shénnóng is also believed to have introduced acupuncture.

The Shénnóng Běn Cǎo Jīng (Divine Farmer’s Herb-Root Classic), compiled between 300 bce to 200 ad from oral traditions, cataloged Shénnóng’s supposed knowledge, which consisted of 365 medicines derived from plant, mineral, and animal sources.

120 harmless herbs were listed in the 1st work, featuring licorice, cinnamon, jujube, ginseng, orange, and reishi. The 2nd volume covered 120 substances for treating the sick, listing cucumber, ephedra, ginger, hemp, opium, and peonies among them.

Chaulmoogra, an oil derived from kalaw trees, was also listed. Chaulmoogra oil was discovered by Western physicians in the 19th century to be an effective treatment for leprosy: a bacterial infection that had been a scourge for thousands of years.

The 125 entries of the 3rd treatise were of violent reaction to ingesting the listed plants, which were usually considered poisonous. Rhubarb, pitted fruits, and peaches were featured.

While various parts of the rhubarb have culinary and herbal uses, the leaves are toxic. The pits of peaches and apricots contain the toxins cyanogenic glycoside and amygdalin. Plum and cherry pits, as well the seeds of pears and apples, contain cyanide. All these toxins are phytochemical protections for plants’ own procreation.

Egyptian scrolls from 1,500 bce listed more than 850 plant medicines. The Egyptian pharmacopeia included aloe, caster bean, mandrake, and opium. Garlic reputedly repelled snakes and discouraged tapeworms.

The slaves that worked the pyramids were fed onions and garlic to ward against infections. Juniper berries from Lebanon were used in purification ceremonies for the dead and wrapped in mummies. Royalty were entombed with various cosmetics, perfumes, and herbal remedies that may be of need in the afterlife.

Also around 1,500 bce, portions of the Hindu Rig Veda chronicled medical advice. This epic Sanskrit poem became the basis for the Ayurvedic system of medicine, with a pharmacopeia that comprises more than 1,500 plant-based remedies.

The Rig Veda mentioned snakeroot to treat snakebite, as a sedative, and for treating mental illness. Used for thousands of years in India, snakeroot first became known to Western medicine in the mid-20th century. Its active ingredient was found to depress central nervous system activity: valuable for treating hypertension and schizophrenia.

 Early Europe

In the late 4th century bce, Greek physician Hippocrates believed disease arose from imbalance in the 4 bodily humors. Hippocrates knew of 300 plant species that might heal. Hippocrates primarily employed purgatives and emetics, which he prescribed to correct internal imbalances by purging offending humors. Hippocrates was mindful of their power, whence the Hippocratic oath.

A century after Hippocrates, Theophrastus became the first Western botanist, writing extensively about plants. His Historia Plantarum (Treatise on Plants) collated his pharmacopeia of plant medicines, spices, and perfumes. It was such a reliable reference that it remained in use for 2,000 years.

Rome’s ascent had many Greek physicians emigrating to parts of the empire where herbal remedies were applied by peddlers, slaves, and elder womenfolk with practical knowledge. Much of Hippocrates’ wisdom was forgotten, but herbal medicine gained official regard upon Julius Caesar’s elevation of Greek physicians to Roman citizenship in 46 bce.

The weed Queen Anne’s lace and silphium were known respectively for their contraceptive and abortive effects as early as Hippocratic times; a feminine knowledge not lost by succeeding generations. Silphium, a relative of fennel, went extinct in the 3rd or 4th century ce, probably from overharvesting.

In the 1st century ce, Greek pharmacologist Pedanius Dioscorides authored De Materia Medica: a 5-volume pharmacopeia featuring 600 medicinal plants; widely read for 1,500 years. Dioscorides was a connoisseur of botanical subtleties. He knew that the aspect of flowering and time of day foretold metabolite potency.

So it is with opium, which has 4 times the morphine if the latex is collected in early morning. Opium poppies have been cultivated for food, anesthesia, and ritual purposes since Neolithic times.

 Medieval Times

The advent of European Christianity meant the abandonment of rational science for church dogma. This included the practice of medicine. It was a throwback to Stone Age times of superstition but more rigorously codified, and contravention barbarically enforced as heresy.

At the close of the 4th century, Christians torched the temple of Zeus at Alexandria in northern Egypt, which housed a medical school, with a library of 700,000 books, including much hard-won medical knowledge. Post-modern Islamic radicals would be the spiritual descendants of such smug ignorance.

In medieval times, medical practice was reduced to barber-surgeons and the ministrations of monks, who were more apt to prescribe purging and repentance than botanical remedies familiar to the ancients and the more practical pagans of earlier times.

Some monastery libraries preserved the classic texts of botany and medicine until they were again allowed as public knowledge. De Cultura Hortorum, a 9th-century poem, described medicinal flora growing in a monastery garden.

Contemporaneously, in the far East, Shénnóng’s classic was being kept up to date. The Arabs were building a new hospital in Baghdad, upon a foundation of translated Greek and Roman medical texts.

Persian polymath Avicenna (980 – 1037) became court physician by age 18. He wrote some 450 treatises. His most famous works were The Book of Healing and The Canon of Medicine. The Canon became a standard text at many medieval universities and remained so for 500 years.

 Doctrine of Signatures

Despite Christian idiocy, reliance on plant remedies remained the norm, being codified in the Doctrine of Signatures beginning ~70 ce. Its naïve approach was a concept borrowed from the Chinese, who conceived that the medical application of a plant could be detected by its signature characteristics: shape, texture, color, and taste.

According to the Doctrine of Signatures, for a plant resembling a certain body part, such as the liver or the heart, its signature signaled its use. Swiss German physician Paracelsus promoted the Doctrine in the early 16th century.

Boneset, eyebright, toothwort, liverwort, maidenhair, and heartsease were names given in view of the Doctrine of Signatures. Eyebrights were daisies, with their brightly colored centers, suggesting application for improving eyesight.

Mandrake in the West and ginseng in the East, though yielding divergent effects, and strong ones at that, were potent under the Doctrine for being suggestive of the whole human body.

Mandrake was used to deaden pain for 2,000 years before ether took over its anesthetic gig. The mandrake plant produces atropine and scopolamine, 2 powerful alkaloids. Scopolamine was used in criminal proceedings as part of a “truth serum.”

Though the Doctrine of Signatures seems far-fetched, its appeal lay in codifying an otherwise cacophony of correspondence between ailment and treatment. The Doctrine spread throughout Europe by oral tradition; its signature remedies affected by some history of success, and thus providing a memory device of at least some merit.

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Fierce faith was placed in the power of plants with aromatic pungency. Those sick and dying of the plague in the 14th century were tended by physicians armored by leather hoods and face cones filled with reeking herbs. Bouquets of pungent wormwood were kept in courts of law to combat infections from prisoners brought into the chambers.

 The Renaissance

The Dark Ages eventually gave way to the Renaissance. In the 1500s came a variety of medical botanical texts, almost always profusely illustrated.

One such herbal work was by German theologian and botanist Otto Brunfels: Herbarum Vivae Eicones (1530–1540), which was illustrated by excellent woodcuts from German artist Hans Weiditz.

Brunfels compiled from earlier texts; a common practice. But Weiditz took a fresh approach: working from live specimens and capturing his subjects as they were, leaving in wilted flowers and bent leaves. Pictured is Weiditz’s woodcut of hellebore, a purgative and poison known from ancient times.

Brufnels was content to write only of plants archaically acknowledged, but Weiditz insisted upon illustrating plants unknown to ancient botanists and physicians. The result was a much more comprehensive, 3-volume body of work.

The work was ultimately flawed by Brufnels’ utter disregard for plant geography. Yet Brunfels has often been called a father of botany for his coverage of plants not known to the ancients (at Weiditz’s urging).

1,800 years earlier, Theophrastus had noted that plants were regional; an ignored truth that resulted in Brufnels books being fraught with discrepancies.

The Doctrine of Signatures continued to hold sway with many, some of whom, including Italian polymath Giambattista della Porta, mixed science with superstition in trying to work out simple formulae for herbal remedies. If plant science failed to sufficiently illuminate a rationale, Porta’s belief in natural astrology guided his medicinal theorizing.

 The 19th Century

Folks with their wits about them knew that advertisements were just a pack of lies – you had only to look at the claims of patent medicines! ~ American novelist Frances Parkinson Keyes

Interest in medicinal plants remained as medical science advanced. American Asa Gray trained as a physician but abandoned medicine for botany, becoming renowned for unifying the taxonomy of North American plants. Gray and Darwin both shared interests in medicine and botany and were lifelong correspondents.

By the mid-19th century, few physicians cultivated their own medicinal herbs. Instead they relied upon suppliers.

The lifestyle-conservative and socially progressive Shakers arrived in America from England in the 1770s. One enduring legacy of this religious sect was its institution of gender equality, unheard-of in the day.

Another Shaker tradition was herbal medicine. By 1826 there were 18 Shaker communities in the United States. Several were in the business of gathering and growing, processing and packaging, and selling herbal treatments exclusively to physicians and pharmacists. Their comprehensive catalogs accurately listed 200 to 400 plants by their common and botanical names, both native American and of European origin, the seeds of which were imported and grown.

The Shakers produced a consistent, high-quality product with their plants, and most everything they made and sold. This was at a time when “patented medicines” were cultivating a bad reputation through aggressive marketing. An 1849 Congressional report criticized these rum remedies as “an evil over which the friends of science and humanity never cease to mourn.” The Shakers’ botanical knowledge and professionalism kept them out of the fray, and even encouraged confidence in their products by favorable comparison to snake-oil remedies.

Organic chemistry was on the scientific horizon at the beginning of the 19th century. It had been long held that a natural vital force was behind the complex chemicals produced by medicinal plants, something that scientists could not hope to reproduce in the laboratory.

German chemist Friedrich Wöhler accidentally synthesized urea in 1828, casting doubt about the truth of there being a vital force of Nature. (That complex compounds may be synthesized does not dispel that there is an intelligent force behind their natural construction, nor does it disprove the veracity of vitalism.) A few years later, Wöhler was among those promoting the doctrine of compound radicals: that chemical groups can readily substitute for elements in constructing complex chemical compounds. This radical theory has been disproven.

 The Rise of Synthetics

Increasing knowledge of chemistry spurred efforts to synthesize those compounds presumed to be active ingredients. Coal and wood tars were the starting point for the early synthetic organic medicines.

The 1820 1st edition of the United States Pharmacopeia (USP) cataloged 650 drugs; 455 (70%) were plant derived. The 1936 11th edition of USP had 570 drugs, with plant products only 40% (206). Between 1920 and 1936, the federal government dropped recognition of many long-standing plant-derived remedies, favoring corporate-produced synthetics whenever available.

The trend to synthetics stalled in part by the discovery of antibiotics. Penicillin was documented in a British publication in 1875, but its discovery, which was entirely accidental, is generally attributed to Scottish pharmacologist Alexander Fleming in 1928. Turning penicillin into medicine was a haphazard endeavor, with the first tangible results in 1942.

The first isolated antibiotic was actinomycin, in 1940, from Streptomyces soil bacteria; though actinomycin’s toxicity precludes its use as an antibiotic. Actinomycin D is one of the older chemotherapy drugs.

Chemotherapy is the great modern medical dice roll of cancer treatment: a painful cellular purgative used as a Hail Mary in hopes of beating the reaper a bit longer. Such treatment illustrates how humans, driven by ignorant fear, value suffering greater than its release.

Plant medicines remain common either because synthesis still eludes researchers (e.g., morphine, cocaine, podophyllin, digitalis) or synthesized versions are cost-prohibitive compared to the natural product (e.g., atropine, reserpine).

The superior knowledge of plants is still heavily relied upon. Even now, 2/3rds of new drugs originate with the discovery of the power of a secondary metabolite.

Plants Synopsis

▫ The chemical energy rumbling in rocks is a leftover from Earth’s energetic origin. Though this energy sustains some microbes, life would have been severely limited had it been restricted to the geological produce of geysers and hydrothermal vents.

By harnessing the inexhaustible energy supply of sunlight, photosynthesis vastly expanded the possibilities for life to evolve. Beyond being a power plant for oceanic life, plants parlayed photosynthesis to establish themselves on land and become the preeminent life there.

▫ Plants are hardy. Plants have persisted, relatively unchanged, across extinction events that radically altered the mix of animal life. When climactic events affected plant populations, they were always quick to make a comeback; such is their savvy adaptability.

▫ Plants manage their own microbiome, which is carefully cultivated. A subset of bacterial and fungal species in the nearby soil cluster about roots; an even smaller subset is allowed inside as a merit reward for productive service.

The same stresses that affect plants affect the microbes that live among them. Microbes help plants adapt more quickly than they could otherwise.

▫ Plant immune systems are more sophisticated than those of animals. Unlike animals, plants are actively aware of an infection at the molecular level, and consciously decide how to deal with it.

▫ Plants are highly intelligent. Plants are self-aware. Plants learn. Plants possess long-term memory. Unlike animals, all plant functions and behaviors are under conscious control.

A plant acquires the collective wisdom of its roots and combines it with the knowledge learned aboveground by branches, leaves, and flowers. From this, a plant fully appreciates its situation in life, and creates social interactions which favor its prosperity.

Plant epigenomes provide an incredibly extensive database of knowledge about numerous life forms, including animals. Unlike animals, plants can consciously tap into and use this knowledge acquired by evolution.

Plants make rational reward/risk decisions, incorporating a richer set of information than animals employ in making judgments.

▫ Plant behaviors comprise chemical concoctions and changing phenotype as well as movements.

Plants have a variety of strategies to optimize health, growth, and propagation.

Plants have a diverse variety of micromanagement techniques to cope with various stresses, including too little or too much water, temperature extremes, and toxic pollution. Plants learn from episodes of stress to better manage future responses.

Most of plants’ abilities to alter cellular chemical compositions and dynamics to deal with various stresses are little understood.

▫ Metabolites are the chemical products of plant metabolism. Primary metabolites are those essential for a plant to live. Secondary metabolites are specialties produced for defensive purposes or as chemical propaganda (such as putting caffeine in nectar to enhance pollinators’ memories).

Many secondary metabolites are poisonous, targeted at specific species. The production of secondary metabolites is timed to correspond with specific need, including accounting for the activities of the pests that prey upon plants.

Secondary metabolites have been the primary source of medicine for humans throughout history.

▫ Plants are adroit chemists, and sheer wizards at understanding the intimate chemistries of other life forms.

Some secondary metabolites demonstrate plants as having intimate knowledge of the life cycle of their predators at the cellular level, such as being able to retard development. Other metabolites change animals’ behaviors. Some act as chemical cries for help to a predator of the pest preying on the plant.

Unlike animals, which rely upon brute force to eradicate viruses, plants can rob invading viruses of their ability to hijack plant cells for replication by knowing exactly what it takes to silence a virus’ RNA. How plants possess such fantastic hidden information to outwit an infection or fend off predation is not known.

▫ Plants are social: recognizing their relatives and adjusting their growth patterns to accommodate kin, while vigorously competing against unrelated vegetation.

Plants have a most varied sociality, including cooperative relationships with bacteria, fungi, insects, birds, and with each other. They also have various stratagems for dealing with pests.

Ever molecularly verbose, plants communicate with one another and with other species. Various advertisements by plants are employed to foster growth and health, eliminate rivals, and attract assistance, such as for pollination, and help killing parasites.

▫ Microbes and plants share the ability to selectively alter their genetic composition, and are thus able to evolve rapidly, and intelligently adapt to changing environmental conditions. It is this coherent flexibility that renders plants the foundations of the world’s terrestrial biosystems, upon which all other land life forms ultimately rely.

Plants’ effect on the planet created conditions favorable for all other macroscopic life. Plants made animals possible. Loss of plant life can only be a harbinger for the fate of animals.