Source: Miller and Levine, Biology
Each part of a seed serves an important function. This image illustrates and identifies the five most important parts of a seed: the seed coat, the endosperm, and the embryo's primary root, cotyledon, and embryonic leaves.
Inside a Seed (long description) (Document)
A seed is to a plant what an embryo is to an animal: an organism in its earliest stages of development. As lifeless as seeds may look before germination, the potential they hold is clear. Simply pick up an acorn and gaze at the oak tree from which it has fallen, and you can see the potential for yourself.
A seed consists of three main parts: the seed coat, the endosperm, and the embryo. Of these parts, the embryo is clearly the most important. Its cells will differentiate and develop into all the different tissues that will ultimately make up the mature plant. The other parts of the seed play merely supporting roles. These roles, nonetheless, are critical to the embryo's success.
The seed coat protects the internal parts of the seed during a period called dormancy, prior to germination. Dormancy is a protected state during which a seed "waits" for favorable growing conditions. Indeed, the seed coats of some seeds allow them to wait a very long time. The oldest known viable seeds were from an East Indian lotus. They were 466 years old when they germinated.
Germination usually begins when the embryo is exposed to water. The water swells the embryo inside, bursting the seed coat and setting growth into motion. During the earliest phase of growth, when the embryo has no leaves and therefore no means of photosynthesis, the endosperm serves as a food source. It serves the same function as the yolk in a bird egg, providing high-energy food to the developing embryo.
The embryo of a seed has three main parts: the primary root, the cotyledon(s) (there are two in many kinds of plants), and the embryonic leaves. The primary root, or radicle, is the first structure to emerge from the seed during germination. It penetrates the soil very rapidly, forming a slender, usually unbranched taproot, which, in some plants, may penetrate several feet into the soil during the first few weeks of growth.
During this period, the cotyledon serves a function similar to that of the endosperm, supplying food to other parts of the developing embryo. Not surprisingly, the embryonic leaves, also known as seed leaves, develop into the plant's first leaves above ground. These leaves open within a few days after the plant emerges from the soil and begin photosynthesizing almost immediately to provide the growing seedling with its new -- and renewable -- food source.
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