Flowering plants fertilise twice: one sperm makes the embryo, the other its lunch
Every seed of a flowering plant is the product of a double act. A pollen tube delivers two sperm cells; one fertilises the egg, while the other fuses with two more nuclei to build endosperm, the embryo's food store. The trick belongs to a group spanning towering gum trees to duckweeds under two millimetres wide.
Angiosperms, named from Greek words for vessel and seed because their seeds sit enclosed in fruit, are by far the most varied land plants: about 300,000 known species in 416 families, with estimates ranging from 250,000 to 400,000. Mosses, by comparison, number around 12,000. Besides flowers, they differ from conifers and their kin in having water-conducting vessel elements and seeds packed with endosperm. Three quarters of species are eudicots and nearly a quarter monocots, leaving tiny early branches with just over 250 species between them.
The pollen grain lands on the stigma and grows a tube down the style into the ovary. Inside, a generative cell divides into two sperm. When the tube reaches the ovule it bursts into a helper cell and releases them: one produces a diploid zygote, the other a triploid cell that becomes endosperm. The ovary then swells into fruit and each ovule into a seed. Many plants add safeguards against self-pollination, from stamens and carpels of different lengths to chemical self-rejection or separate male and female plants, as in holly.
Their ancestors split from gymnosperms over 300 million years ago, but flowering plants only burst onto the scene in the Early Cretaceous, about 130 million years ago, then rapidly took over habitats held by ferns and conifers. Claimed older flowers remain unaccepted, many coming from a single research group. Partnerships with insects, birds and bats drove the explosion; flowers evolved nectar, colours and scents, and shapes that fit particular pollinators, like tubes for hummingbirds, which in turn split populations into new species.
About 99 percent photosynthesise, while the rest parasitise fungi or other plants. They dominate every land habitat except cold moss-lichen tundra and conifer forest, and seagrasses even colonise the sea. Rice, maize and wheat, all grasses, supply half the world's staple calories.
Source: Flowering plant
