Seedling

Seedling

At germination the testa is ruptured and the seedling radicle emerges through the micropyle and pushes through the substrate. Seedlings possess a root (radicle) and a hypocotyl, which bears the cotyledons (seed leaves) and shoot apex. The hypocotyl varies in size and form, from a swollen food-storage organ to a very short structure which may be almost non-existent, in which case the radicle extends almost to the cotyledonary node. Following the emergence of the radicle, either the hypocotyl elongates and the cotyledons and shoot apex emerge (termed epigeal germination),or the cotyledons remain enclosed in the testa and the internode above them (the epicotyl) elongates, pushing the shoot apex upwards (termed hypogeal germination). Epigeal germination is the most common type in angiosperms; the cotyledons are borne above ground, and are usually photosynthetic. By contrast, some larger-seeded eudicots such as legumes (e.g. Vicia faba) are hypogeal, and possess fleshy, swollen cotyledons.

In monocots the radicle withers at an early stage, and subsequent roots are shoot-borne (adventitious); they are each initially surrounded by a sheath (coleorhiza), which develops from outer cortical tissue by cell elongation. The cotyledons are usually morphologically different from the first foliage leaves; they typically possess simpler vasculature which often consists of a single vascular bundle. In monocot seedlings the cotyledon typically includes three parts¹⁰⁹: a basal sheath, a ligule or ligular sheath, and a limb, though the relative differentiation of each region varies; for example, in Tigridia seedlings both the hypocotyl and the basal sheath are extremely reduced.