Hypocotyl

The hypocotyl is the portion of a seedling stem located between the cotyledons (seed leaves) and the radicle (embryonic root). In most dicotyledonous species it forms the short axis that connects the shoot system bearing the cotyledons with the developing root system. In monocots, the term is used less consistently because grasses often have a coleoptile protecting the shoot apex and a relatively short or inconspicuous hypocotyl; in some species the hypocotyl remains subterranean during germination (hypogeal).

During germination, the hypocotyl often plays a key role in establishing the young shoot. In epigeal germination, the hypocotyl elongates and lifts the cotyledons above the soil surface, exposing them to light. In hypogeal germination, the cotyledons stay below ground and the hypocotyl elongation is reduced, with the shoot emerging mainly from the epicotyl region.

Anatomically, the hypocotyl serves as a conduit and support structure. It contains vascular tissue that connects the cotyledonary vascular bundles with the root stele, aiding the transport of water and nutrients as the seedling develops. It also provides mechanical support to hold the young plant upright until true stems and leaves can take over growth.

Physiologically, hypocotyl growth is regulated by plant hormones, particularly gibberellins and auxins, which promote elongation, while abscisic acid can inhibit germination. Etiolation in darkness produces elongated, pale hypocotyls with limited chlorophyll, reflecting adaptations to low-light conditions.

Etymology and significance: hypocotyl comes from hypo- 'under' and kotyl- 'cup,' reflecting its position beneath the cotyledons. Understanding hypocotyl development is important for seedling vigor, germination ecology, and crop establishment.