A seed is a plant in its earliest stage of development. Seeds are classified based on the number of cotyledons, or seed leaves, they contain, with monocots having a single cotyledon and dicots having two. While not all seeds on the planet have the exact same structure, most contain the same basic parts: an embryo, a protective outer coating, and an internal food source.
The seed coat is the outer covering of the seed. It helps protect the plant embryo from drying and injury. The covering also helps prevent early germination by keeping water outside the seed, and makes it more difficult for insects to eat or damage the embryo. Seed coats vary in thickness and texture, depending on the plant.
The endosperm functions as the seed's food source. Because the seed has no access to sunlight, it cannot engage in photosynthesis and therefore cannot manufacture its own source of energy. The endosperm is formed when fertilization occurs, and is generally used up in entirety by the time germination begins.
The embryo is a tiny version of the plant in a dormant state. It is the most important part of a seed, as it is the part that matures into the final plant. The embryo consists of a radicle, a hypocotyl or epicotyl and one or two cotyledons.
Upon germination, the radicle develops into the primary root, growing rapidly and penetrating the soil. The cotyledons are typically the first part of the seed visible upon germination, according to the University of Illinois, and act as a food source for the growing embryo. They develop into the plant's first leaves after only a few days above ground. The hypocotyl or epicotyl functions as the plant's stem after germination.