hypocotyls

The hypocotyl is the portion of a seed plant’s embryo axis that lies between the radicle (future root) and the cotyledons. In germinating seeds, it forms the first stem segment that connects root and shoot and often guides the young seedling’s emergence through soil.

Germination can be epigeal or hypogeal. In epigeal germination, the hypocotyl elongates and raises the cotyledons above the soil surface. In hypogeal germination, the cotyledons remain below ground and the epicotyl or shoot axis enlarges, while the hypocotyl remains comparatively short. In monocotyledons such as grasses, the shoot often emerges through a protective coleoptile, and the hypocotyl may be less prominent than in many dicots.

Anatomically, the hypocotyl consists mainly of parenchyma with supporting tissues and vascular connections linking the radicle to the cotyledons and the developing shoot. The region is typically transient, eventually becoming part of the mature stem as the seedling establishes photosynthetic growth.

Significance in research and agriculture centers on seedling vigor and early growth responses. Hypocotyl length is a common parameter in studies of photomorphogenesis, etiolation, and seedling establishment. Etiolated seedlings grown in darkness exhibit elongated hypocotyls, whereas light exposure generally promotes shorter hypocotyls and more developed cotyledons and leaves.