Every plant seed contains all the parts it needs to grow into a plant. There are three main parts of a plant seed: the seed coat, the embryo and the endosperm. The embryo itself contains three separate components, consisting of the cotyledon, the embryonic leaves and the primary root. Each seed component has its own specific purpose and function in developing into a mature plant.
The seed coat (testa) is the outer layer of the seed, which protects the inner parts of the plant seed. The seed coat enables the seed to remain in a dormant state before germination so that the seed begins to grow during the most favorable environmental conditions. When the seed coat is exposed to the right temperature and moisture, the internal embryo will swell and burst the seed coat.
The endosperm provides food to the plant seed before and during germination. In mature, germinating seeds, the endosperm is used up and disappears. The endosperm essentially provides food to the seed until the leaves emerge and are able to begin photosynthesizing.
The cotyledon is part of the seed embryo, and there are two in most plant species' seeds. The cotyledon supplies and converts the food from the endosperm to the other parts of the embryo during germination and development of the seed. Plants with two cotyledons, such as beans, are called dicots, while those with one cotyledon, such as corn, are called monocots.
The primary root is also part of the embryo. Also called the radicle, the primary root is the undeveloped plant root. This is the first part of the seed to emerge through the seed coat during germination.
The third embryo component, the embryonic leaves are the "seed leaves." The embryonic leaves grow into the plant's first set of leaves after it germinates and sprouts above the soil. The embryonic leaves are essential to the plant growing beyond the first germination stages, because these seed leaves allow the plant to begin photosynthesis right away.