Seed

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Seed, term applied to the ripened ovule of a seed plant before germination. Seeds of the angiosperm, or flowering plant, differ from those of the gymnosperm, or conifer and related plants, in being enclosed in the ovary that later forms a fruit; gymnosperm seeds lie exposed on the scales of the cones.

During the process of fertilization the pollen tube enters the ovule through a small opening known as the micropyle. One of the two sperm nuclei in the pollen tube unites with the egg cell in the ovule to form a zygote, which develops into the embryo. In flowering plants the other sperm nucleus unites with two polar nuclei present in the embryo sac to form an endosperm nucleus, which later produces the nutritive endosperm tissue surrounding the embryo in the seed. In gymnosperms, the endosperm is formed from the tissue of the embryo sac itself. The nucellus, or megasporangium, is the tissue composing the main part of the ovule; it is partially digested during the development of the embryo and endosperm tissue. Surrounding the seed is a hard, tough seed coat, derived from the integument of the ovule and known as the testa. In flowering plants a second seed coat occurs within the testa; this second coat is thin and membranous and is known as the tegmen. Some seeds, in addition, have projects from the seed coat that serve to aid in the absorption of water when the seed is about to germinate or that merely form an additional protective coating about the seed. In almost every seed, the micropyle through which the pollen tube entered the ovule persists as a small opening in the seed coat. Close to the micropyle in flowering plants, a stalk, or funiculus, attaches the seed to the placenta on the inside of the fruit wall. When the seed is removed, a small scar, known as the hilum, marks the former attachment of the stalk.

See also Gymnosperm and Angiosperm Seeds; Monocot and Dicot Seeds.

SEED GERMINATION

Most seeds begin to germinate only with the warming days of spring, months after they have fallen to the ground. As the embryo inside expands, the seed cracks, and a root emerges to provide the seedling with both stability and nutrients from the soil. While the root continues to grow and branch downward, the embryonic stem sprouts upward. Nourished from this point by the cotyledons, or seed leaves previously folded within the seed coat, the seedling will develop a shoot with adult leaves.

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