The Elements of Evolution (19-2) Flowering Plants continued


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