germination

Dormant seeds are very dry and require the absorption of water to initiate the metabolic processes of respiration and begin to digest their stored food. Respiration requires the presence of oxygen, which must be sufficiently available in the soil for germination to proceed, so the soil must be wet but not so waterlogged as to make oxygen inaccessible. Temperatures must be above freezing (zero degrees Celsius) but not excessively hot (not more than about 45 degrees Celsius). If conditions are right, a radicle (an embryonic root) emerges from the seed coat, anchoring the seed; it then grows and puts out lateral roots. In most eudicots, a part of the developing stem, either the epicotyl (the stem above the cotyledons) or the hypocotyl (the stem below the cotyledons) elongates, forming a hook and gradually pulling the seed coat and the delicate shoot tip above the soil surface. Germination of eudicot seeds is normally divided into two types, designated epigeous and hypogeous. In epigeous germination, the cotyledons emerge above the soil surface, and wither and drop off after their food stores have been used up; in hypogeous germination, the cotyledons remain below the surface and decompose after their food stores have been used up. In most monocots, food is stored in the seed's endosperm (rather than the cotyledon), and it is the single tubular cotyledon that elongates and draws the seed coat out of the soil. The cotyledon conducts photosynthesis, making more food, while the shoot grows up inside the tube.