The seed, which surrounds the plant embryo and protects it from desiccation in the uncertain conditions of a terrestrial environment, is one of the adaptations that has allowed plants to thrive as they gradually moved from water to land. Each seed consists of an embryo, food source, and protective outer coat; it can lie dormant for some time before germinating, waiting until environmental conditions are right.
The two classes of seed-bearing plants are gymnosperms (conifers) and angiosperms (flowering plants). The composition of the seed varies by type of plant. As discussed in Plant Classification, Gymnosperms, the gymnosperm ovule develops into the seed after fertilization takes place. The tissue of the female gametophyte within the ovule, where the egg cell was generated, surrounds the developing embryo and becomes its food source. The integument (outer surface) of the ovule becomes the seed coat.
The seeds of angiosperms are somewhat different from those of gymnosperms (see Plant Classification, Angiosperms. In angiosperms, the food source for the seed is derived (in a process called double fertilization) from a triploid nucleus that is formed during fertilization alongside the creation of the zygote. This triploid nucleus gives rise to a starchy substance called endosperm, which nourishes the developing embryo, and, in dicots, the seedling. Angiosperm seeds, which develop from ovules, are also distinguished from gymnosperm seeds because they are enclosed in protective ovaries. These ovaries, which are derived from carpels (modified leaves) on the flower, develop into fruit after fertilization. The fruit provides added protection against desiccation of the embryo and aids in its dispersal by wind and animals.
As the angiosperm seed develops, so do the cotyledons, or seed leaves. Monocot embryos have one cotyledon, while dicot embryos have two. At the same time, embryonic "shoot" and "root" develop as well; taken together, they are called the embryonic axis. The part of the embryonic axis located above the point of attachment of the cotyledon(s) is called the epicotyl, and is destined to become the shoot. The embryonic axis below the cotyledon attachment site is called the hypocotyl, and is the progenitor of the root.