Seeds are marvelous packages. They contain the beginnings of new life, along with the food needed for that new plant to grow, all wrapped up in a protective covering. Seeds are classified as either monocot or dicot, which simply refers to the fact that the inner embryo has either one or two leaves. Both types of seeds have three main parts, each of which contains several other components according to Holly S. Kennell, a horticulturist with Washington State University.
The seed coat is the outer covering of the seed. Usually smooth and hard, it is there to protect the tiny embryo from environmental conditions and insect pests. The seed coat is either thin or thick. Thin seed coats easily germinate (split open) when the conditions are right (sun and moisture are present). Thick seed coats are present on seeds that are usually consumed by animals and then excreted. The coat becomes thinner during the process of digestion. If these thickly-coated seeds are not consumed by animals, they usually must be soaked in water for a couple of days, or rubbed with sandpaper to soften them up in order for them to germinate.
The endosperm, which is found just beneath the seed coat, makes up the bulk of the seed and provides food to the embryo as it lies dormant and is waiting for germination. This can take a very long time -- even over 100 years.
Once the seed sprouts, the cotyledon begins to provide food to the new plant. The cotyledon forms a barrier between the embryonic leaves and roots and the endosperm. It is considered to be part of the embryo.
Embryo and Primary Root
The embryo lies curled up in the seed and contains the cotyledon mentioned above, as well as the embryonic leaf or leaves and primary root. As water penetrates the seed coat, the embryo swells, and the primary root (called the radicle) emerges and penetrates deeply into the soil.
Sprout (Embryonic Leaves)
The second part of the seed to emerge is the embryonic leaf (sprout), which grows upwards toward the sun. The cotyledon feeds the embryo for up to 10 days until the leaf or leaves appear above ground, unfurl, and begin photosynthesizing.