Seed Germination

Germination is the sequence of physiological events by which a mature, viable seed -- until now metabolically quiescent -- resumes active growth and gives rise to a self-sustaining seedling. It begins with imbibition, the uptake of water by the dry seed coat and embryo, which rehydrates the cytoplasm, activates stored enzymes (chiefly hydrolases that mobilise the food reserves packed into the endosperm or cotyledons as starch, oil and protein) and switches on respiration and cell division in the embryo. In almost every species the radicle (embryonic root) is the first structure to emerge through the seed coat, anchoring the seedling and beginning to draw up further water, followed shortly afterward by the plumule (embryonic shoot).

What happens next to the cotyledons, however, follows one of two contrasting patterns, and this distinction is the single most useful practical marker for classifying a seedling's germination type. In epigeal germination, the hypocotyl -- the short stretch of embryonic axis lying between the radicle and the point where the cotyledons are attached -- elongates rapidly and, because it is anchored below by the already-growing root, arches upward and carries the cotyledons bodily above the soil surface. Gram (Bengal gram), castor and sunflower germinate this way: their cotyledons emerge into the light, often turn green and photosynthesise briefly, and only wither once the true leaves have taken over food production. In hypogeal germination, by contrast, it is the epicotyl -- the region of the axis above the cotyledons -- that elongates, so the cotyledons themselves stay put below the soil surface while only the plumule pushes upward into the light. Maize and other grasses germinate this way, with the single cotyledon (called the scutellum in grasses) remaining underground and the young shoot protected as it pushes through the soil by a sheathing structure called the coleoptile; pea and mango are common hypogeal examples among dicots. Neither pattern is 'better' than the other -- they are simply two different solutions, both common across flowering plants, to the same problem of getting a young seedling established and photosynthesising as quickly as possible after germination begins.