What is a seed?

(Click to enlarge)

Nowadays everybody knows that a plant comes from a seed, and that a seed comes from a plant.

Botanically speaking, a seed is the result of the sexual reproduction of a plant. When the flower is fertilized, seeds are produced inside the fruit which protects them and grows bigger. When ripe, a dry fruit will open and let the seeds out; a fleshy fruit will have to be digested by animals or have to get rotten before seeds can be released. Each year, tons of fruits splash on the earth, and millions of seeds rain on the ground. This is how plants reproduce.

Almost every part of the earth's surface is filled with seeds or vivarious roots of seedlings of various kinds, and in some cases probably seeds are dug up from far below the surface which still retain their vitality. The earth is a granary and a seminary, so that to some minds its surface is regarded as the cuticle of one great living creature. - Henry D. Thoreau

Let's look closely at a seed. For instance, let's open a bean:

What we see at first are these two white parts which is mainly what we eat. They are called "cotyledons", they are full of starch, which is the compound standing as nutritive reserves for the seed. We also distinguish a small "germ". By looking closer (zoom), we will be able to see that it is a tiny miniature plant: it is a baby plant.The whole seed is coated by an envelope called "testa". Its function is very important: it has to protect the baby plant and also to receive and interpret all the informations coming from the outside world. It is the testa which receives the signal to start the germination process.