Seed Structure and Development
Following the fertilizations in the embryo sac, the zygote divides repeatedly by mitosis and differentiates into an embryo. The endosperm nucleus also divides by mitosis and forms the endosperm tissue, which provides food for the developing embryo.
The early embryo is linear with apical meristems on either end and one or two seed leaves or cotyledons. The axis below the cotyledons is called the hypocotyl, at the tip of which is the radicle that gives rise to the primary root of the seedling. The axis above the attachment of the cotyledons is the epicotyl, which also ends in an apical meristem. In some seeds, the first foliage leaves are formed in the seed. The area above the cotyledons is thus a miniature shoot and is called the plumule. In some taxa, food for the embryo remains within endosperm tissue and the cotyledons serve as organs of absorption. In others, food moves directly into the embryo and is stored within the cotyledons, leaving only a minuscule endosperm. In still another variation, the nucellus (the megasporangium wall) enlarges and becomes a storage tissue called perisperm. The integuments harden into the seed coat as the embryo matures. The scar left on the seed coat by the separation of the funiculus from the integuments is called the hilum. Often the micropyle remains visible near the hilum.
Angiosperms traditionally have been separated into two major categories on the basis of the number of cotyledons they possess: monocots (mono = one; cotyledons = seed leaves) and dicots (di = two; cotyledons = seed leaves). Monocots are the grasses, sedges, lilies, and their relatives, while dicots constitute the rest of the flowering plants. This artificial separation currently is being replaced by a more natural classification in which the monocots are retained as a natural group, but the dicots are separated into the eudicots (eu = true, dicots) and the magnoliids. The latter is a small group of very primitive angiosperms ancestral to both monocots and eudicots and has both woody representatives— woody magnoliids (trees like Magnolias, tulip trees, and laurels)—and paleoherbs (herbaceous plants such as members of the pepper and water lily families). The eudicots constitute about 97 percent of the angiosperms, while the magnoliids make up the other 3 percent.
In monocot seeds the single cotyledon usually digests and absorbs food from the endosperm and translocates it to the embryo. In grasses, the cotyledon is called the scutellum. In grasses, also, there is a protective sheath called the coleoptile over the plumule and another, the coleorhiza, surrounding the radicle.
Internal signals shut off growth of the embryo at a certain size and the seed goes into a period of dormancy. In some plants, dormancy lasts only as long as it takes the seed to be dispersed from the ovary. In others, dormancy may last for long periods until either external or internal signals (or a combination of both) initiate further growth. Environmental factors of chief importance to initiate growth are water, light, and temperature.
When an embryo resumes growth, stored food provides the energy for seedling development—the roots first, followed by elongation of the photosynthetic shoots.
Eudicot development. Aboveground growth in eudicots takes one of two general patterns: epigeous or hypogeous. In epigeous growth, the hypocotyl elongates, pulling the plumule and cotyledons above ground; in hypogeous growth, the cotyledons remain below ground because the epicotyl grows faster than the hypocotyl and pulls the plumule erect.
Monocot development. Monocots develop with two different general patterns: one for the grasses, one for the rest of the group. In most of the monocots (but not grasses), after the radicle has pushed out of the seed coat, the first shoot structure to emerge is the cotyledon, which arches upward with the remainder of the endosperm and the seed coat still attached. It elongates above ground and is photosynthetic until the true leaves develop.
In the grasses, the sheaths around both the shoot and the root tip must be penetrated by the roots and the shoots. The root sheath, the coleorhiza, grows faster than the radicle for a short time, but when it stops growing, the radicle emerges and forms an anchoring primary root. The shoot sheath, the coleoptile, moves upward to the soil surface through elongation of the first internode of the stem (called the mesocotyl), and when it reaches the surface it stops growing. The plumule then pushes through into the air. At about the same time, buds of adventitious roots begin to grow and, by the time the seedling is erect with a few true leaves, it already has adventitious roots growing downward from its first node. The primary root system is short lived and dies soon after its establishment and the adventitious root system becomes the principal absorbing and anchoring system for the new grass plant.