Seeds develop from the ripened ovule of plants after it has been fertilized by the pollen from the male plant. Their shape resembles that of the mother ovule while their size varies as they develop.
A seed contains three parts: an embryo, a supply of food for the embryo and a seed coat.
An embryo is a stem-like organism which springs from a zygote (an egg fertilized by pollen). It comprises the cotyledon or seed leaf. This leaf seed needs water and warmth to begin germination. So it absorbs water and swells giving birth to the radicle or the embryonic root and then to the plumule or the embryonic shoot. The embryo within the seed can survive for long periods without additional food or water resuming its growth when conditions become favorable again.
The food for the embryo can be found either in seed leaves or outside the seed leaves. Its form varies from plant to plant depending on their growth process. Most seeds contain a tissue called the endosperm, rich in oil or starch and protein. It serves several purposes depending on the species of plants and their internal functions.
The seedcoat or the testa develops from the integument surrounding the ovule and serves a a protective shield against outside aggression or dying.
There are two major types of seed plants which complete their life cycles in completety different ways: the angiosperms and the gymnosperms. The angiosperms are kinds of plants which have both male and female parts which, by means of pollination, ensure the birth of new seeds. The male pollen is dropped by birds or insects on a female stigma. From there it gets into the ovary where after a series of transformations it becomes a fertilized egg and then an embryo. The embryo, along with the endosperm that nourishes it, is protected by the testa. But what most differentiates angiosperms from gymnosperms is the phenomenon of double fertilization: angiosperm reproduction is double fertilization: besides egg cell fertilization, central cell nucleus fertilization occurs as well.
Gymnosperms or flowerless plants such as conifers have a rather different cycle of seed development. They are spore-bearing plants (sporophytes) and they are different from the angiosperms through their reproduction method: they use only one sperm cell whilw the other one is left to wither and die.
Seed dormancy is a characteristic of some seeds which do not germinate even under friendly environmental conditions. These seeds adapt easily to harsh conditions and can be cultivated in areas around the globe including Northern Europe or Australia.