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.
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