To create a little flower is the labour of ages. ~ English poet William Blake
280 MYA the proliferation of a tremendous variety of seed plants was well underway. The next milestone in plant evolution came with pollen and flowering plants (angiosperms) in the mid-Triassic, some 245 MYA. This was a divergence from simpler seed-bearing plants.
As always, geology and climate combined to create conditions conducive to more sophisticated plant life. Angiosperms may have got their edge by developing drought-resistant features and rapid reproduction. The warm and moist climate then helped ensure success, though their proliferation took time. Angiosperms diversified enormously during the early Cretaceous, around the same time that many new types of dinosaurs became prominent.
The adaptive advantage of animal pollination is being able to cross-fertilize using a relatively small amount of pollen compared to the copious quantities needed when relying upon the wind, as gymnosperms do. The evolutionary outgrowths were billboards advertising fine dining: flowers.
Insects drove angiosperm speciation, as plants aimed at captive audiences for their delicacies. Until their co-option insects had been a pest to plants. By the time angiosperms arose, the 6-legged creatures were well-known to flora. It was a matter of wiles to turn select insects from foe into ally.
Plants underwent rapid adaptations from interacting with both insect pollinators and predators. Insects too underwent changes to take better advantage of the herbal bounty, whatever feeding off a plant meant.
Attempted insect pollination was not always a success. Several groups of flowering plants went back to the wind, some investing instead in defense. Plants in the nettle family are exemplary. Mulberry family members gave up on pollinators, but some then gave it another go. Certain figs managed a mutualism with little wasps. (While most fig wasp species act as pollinators, some simply feed off the plant. Insects as allies is tricky.)
Though insects drove diversity, dinosaurs also made a dent. High-browsing herbivores, such as sauropods and stegosaurs, were the norm 160 MYA, dieting on a wide range of conifer tissue. This put pressure on the canopies of mature trees and egged on development of plant defenses. During this time, tender saplings went relatively ungrazed. Their relatively modest physical presence gave angiosperms an edge.
Flowering plants revolutionized terrestrial ecosystems. They have a broader range of growth forms than all other plant groups – from giant trees to tiny annual herbs – and can produce nutrient-rich tissues at a faster rate than other plants. So, when they started dominating ecosystems, they allowed for a wider variety of life modes and also for much higher ‘packing’ of species with similar ecological roles, especially in tropical forests. ~ Swedish botanist and paleobiologist Caroline Strömberg
Well over 150 MYA, some in the apoid lineage of wasps began to change their foraging preferences: abandoning hunting in favor of flowers. Bees evolved to rely entirely on tantalizingly sweet nectar, rich in carbohydrates, and protein-rich pollen.
Early bees resembled their carnivorous wasp cousins, with sleek bodies and short tongues (proboscises). Enticed by nectar and pollen, bees evolved longer tongues. Seeking a captive pollinator to improve fertilization productivity, some flowers developed longer flower tubes that only certain bees could access, as well as specialized signage and accoutrements that attracted the desired clientele. This and other traits occurred in radiation and coevolution of bees and flowers.
Bee bodies became hairier, enabling more efficient pollen collection. Some bees, including honeybees, developed specialized structures, such as brushes and pollen baskets.
Coevolution resulted in some stunning specializations. With a straw-like proboscis, the southeastern blueberry bee feeds mostly on blueberry flowers. This bee’s secret for success comes with its buzz: rapidly vibrating against flowers’ anthers to “buzz” off the pollen, which sticks to the forager’s furry body. The captured pollen feeds brood back home as well as pollinating the next blueberry plant. Mutualism evolved to the extent that these bees are active only in early spring, when blueberry blossoms are abundant.
Behind the fragility and beauty of flowers were tough-minded innovations. Angiosperms developed self-incompatible alleles to preclude inbreeding via autogamy (self-fertilization), and so take advantage of the adaptive power of outcrossing.
Also distinctive in angiosperms is double fertilization: a complex reproductive process in which a pollen grain with 2 male sperm (gametes) fertilizes a female embryo sac (gametophyte), with 1 sperm producing the embryo, and the other producing the endosperm, upon which the embryo feeds for its initial growth.
Angiosperms were also adopters of earlier experiments which had proved their worth. Gymnosperms in the desert Ephedra and tropical Gnetum genera employ a rudimentary form of double fertilization. The development of an embryo-nourishing endosperm – an angiosperm hallmark – occurred later. In providing built-in food storage, endosperm yields a critical survival edge when life is at its most tenuous.
As the term angiosperm implies – seed vessel – seeds often have covers which engender animals or the elements to disperse: fleshy, luscious fruity tissues, adhesive burs, feathery parachutes, and other devices. Here are creative solutions for being carried afield. Less impressive but also important are seed coats which protect their wearers against the elements and predation.
Invisible but essential is the precocious intelligence of seeds to know when to sprout. Seeds exist in suspended animation, opportunistically awaiting the moment when the germinating odds may be in their favor.
Beyond the acumen behind individual seeds is a pecuniary consideration: the probability of payoff. Aiming for wide dispersal puts a premium on small size and large numbers. Conversely, crafting a competitive embryo emphasizes large seeds at the expense of numbers. On the whole, the size and number of seeds a plant produces are a compromise of evolutionary considerations which encompass the subtleties of the habitat.
A cotyledon is the embryonic leaf in a plant seed. The earliest angiosperms had 2 such leaves (dicotyledons).
Dicots have a main root (radicle) from which secondary roots may grow. The veins on their leaves are a network (reticulated). Dicot stems arrange their vascular bundles in concentric circles, affording secondary growth and thereby the development of bark that is characteristic of trees.
Economical monocots arose 150 MYA, with superior flexibility at the expense of size. These lithe plants begat the beautiful flowers with which we are so familiar, and the seed plants which we eat.
Monocot seeds have a single seed-leaf, opportunistic roots, flexibly arranged vascular bundles within stems, and the optimization of parallel leaf veins. Able to seize the moment, monocots are quick growers.
A few monocots are capable of secondary growth, as exemplified by palm trees and bananas. Other monocots manage considerable height without it. Sugarcane is a monocot. So too bamboo: the largest member of the grass family, and one of the fastest-growing plants.
From 144 MYA, around the Jurassic-Cretaceous boundary, herbivores changed considerably. Low-browsing ornithischian dinosaurs arose to take advantage of chemically unprotected seedlings. They thinned the forest, creating gaps in the canopy which led to more distributed plant communities.
Embracing their opportunity, monocots were the right plant at the right time: small, short life cycle, and quick colonizers. These wily adventurers took advantage of conditions to radiate their existence and engender allies.
Dicots were not idle while their cousins took the fields. Eudicots, which arose 115 MYA, were an evolutionary advance for dicots: there were optimizations for flowers and pollen, enhancing reproductive success. These innovations independently evolved multiple times.
Eudicots also improved succulence in plants, to better retain water during times of shortage. Eudicots were so successful as to become 75% of all angiosperms.
Over evolutionary time, it’s as if plants have actively explored the best strategies to safeguard their own survival. ~ American evolutionary biologist Lars Hedin