Hypocotyl is:
Part of embryonal axis below cotyledons
The hypocotyl is the part of a plant embryo or seedling that lies between the radicle and the cotyledons. Upon germination, the hypocotyl pushes the cotyledons above the ground to develop. It eventually becomes part of the plant stem. Most seed-bearing plants have hypocotyls, but the grasses have different, specialized structures.