Some 300,000 species strong, flowering plants dominate terrestrial landscapes, prompting awe and wonder dating from the days of Charles Darwin about how this group arose. (See this month's Origins essay.) And although the blossoms themselves contributed much to the success of angiosperms—attracting and making efficient use of insects and other pollinators—they are not the only feature that gave angiosperms a leg up on earlier seed plants. The way angiosperms provide nutrients for their seeds represents an underappreciated, cost-saving innovation that arguably made human civilization possible. It’s called the endosperm, and it's what pops in popcorn (left) and forms the bulk of what's in flour, rice, oats, and other grains, providing humans with two out of every three calories worldwide. “You take endosperm off the table, and you have fern fiddleheads and not much more [to eat],” says William "Ned" Friedman, a botanist at the University of Colorado, Boulder. “Even a grain-fed cow is really processed endosperm.”
Endosperm is the nutrient-filled tissue that sustains the growth of the embryo within a seed, sometimes early in development and sometimes later, once the seed has germinated. It arises through an innovation called “double fertilization.” Each pollen grain produces two sperm, one of which fertilizes the “egg” and one of which merges with a so-called central cell that’s colocated with the “egg” in the female embryo sac. That latter fertilization sets off the growth and development of endosperm, which becomes packed with starches, proteins, lipids, or oil, depending on the species. Endosperm can be solid, as in wheat, or liquid, as in coconut milk. This double fertilization ensures that the embryo will have the sustenance it needs to complete its development.
Pines, firs, ginkgoes, and other gymnosperms also have nutritive tissue, but their tissue is prepared well in advance, laid down as the egg is maturing and well before fertilization. “It’s a slow process,” says Friedman, and should subsequent fertilization fail, this effort goes to waste. By jump-starting embryo and endosperm development simultaneously, angiosperms save time and cut down on wasted effort. “My sense is this [change] opened up all kinds of opportunities,” he adds. For example, now life cycles could be completed in a season, making possible annuals and other rapidly reproducing plants. But how did this just-in-time provisioning evolve?
Photo Credits: William "Ned" Friedman.