Hypocotyl

The embryonic stem region between the cotyledons and the radicle. During germination, it may elongate and lift the cotyledons above the soil.

Hypocotyl elongation: The increase in length of the stem region between a seedling’s cotyledons and radicle. Elongation can lift cotyledons above the soil during germination.
Radicle: The embryonic root that develops into a seedling’s primary root. It marks the lower boundary of the hypocotyl in the embryo.
Hypocotyl grafting: A grafting method that joins plant parts through their hypocotyls, commonly used in vegetable seedlings. The hypocotyl provides the stem segment where rootstock and scion can be joined.
Epigeal germination: Germination in which the cotyledons are raised above the soil surface. Hypocotyl elongation commonly lifts the cotyledons in this germination pattern.
Apical hook: A curved bend near a seedling shoot tip that protects the apical meristem as it emerges from soil. The hook forms in the upper hypocotyl and straightens after emergence.
Cotyledon: An embryonic leaf that may store or absorb food and is often the seedling’s first leaf. Cotyledons mark the upper boundary of the hypocotyl.
Seed germination: The process by which a viable seed resumes growth and develops into a seedling. Hypocotyl extension is a visible part of seedling emergence in many species.
Hypogeal germination: Germination in which the cotyledons remain below the soil surface. Unlike epigeal growth, this pattern does not lift cotyledons above ground.
Photomorphogenesis: Plant development regulated by light, including changes in seedling shape and gene expression. Light exposure suppresses hypocotyl elongation and helps seedlings switch to above-ground growth.
Epicotyl: The embryonic shoot region above the cotyledons, which includes the shoot apex. It is the stem region above the cotyledons, unlike the hypocotyl below them.

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CeleriacRelated: The enlarged edible base is commonly described as including swollen stem tissue.