Seed
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 em...