Seeds packaged plant embryos for a drier planet
A botanical seed wraps an embryo and food stores inside a testa coat; loosely, anything sown can be called seed. Seed plants define themselves by this package, while ferns and mosses still need water-dependent spores. Flowering ovaries ripen into fruits that advertise or hurl those units abroad.
A seed forms when a fertilised ovule ripens, and producing one is what defines the seed plants, or spermatophytes. Ferns, mosses and liverworts manage without, relying on water to spread. The payoff shows in the landscape: seed plants now dominate land habitats from forest to grassland, in hot climates and cold. Land plants first appeared about 468 million years ago and spread by spores; the earliest seed bearers were gymnosperms, which arose in the late Devonian, between 416 and 358 million years ago, followed by seed ferns in the Carboniferous.
Flowering plants enclose their seeds in a fruit grown from the ovary, which both protects and disperses them, and many things sold as seeds, such as sunflower seeds in the shell, are really dry fruits. Gymnosperms, whose name means naked-seeded, have no ovary, so their seeds develop exposed on cone scales. An angiosperm seed combines three genetically distinct parts: an embryo from the zygote, a usually triploid endosperm food store, and a coat made from the mother plant's tissue. It begins with double fertilisation, in which two male gametes fuse separately with the egg and with other cells, whereas in gymnosperms one sperm is normally wasted.
The ovule's anatomy leaves its mark. A stalk called the funicle ties it to the placenta, the pollen tube usually enters through a tiny pore at its tip, the micropyle, and the chalaza lies at the opposite end. Most ovules are curved, a form called anatropous, while straight orthotropous ovules yield uncurved seeds. The coat itself develops from two integuments: the outer becomes the testa and the inner the tegmen.
Inside, the embryo carries one seed leaf in monocots or two in dicots, and in dicots without endosperm those leaves double as the food supply. Above them the epicotyl ends in the feathery plumule, the future shoot; below, the hypocotyl leads to the radicle, the first root. Monocots such as corn add protective sheaths, the coleoptile and coleorhiza. Scientific study of seed storage began early: Augustin de Candolle published a guide in 1832 and germination-lifespan work in 1846.
Source: Seed
