Hypocotyl

hypocotyl [′hī·pə‚käd·əl]

(botany)

The portion of the embryonic plant axis below the cotyledon.

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Hypocotyl 

in sprouting plants, the section of the stem extending from the root collar (the zone between the main stem and the root) to the place of attachment of the cotyledons. In the embryo, the hopocotyl is the rudimentary stem. The sprouts of some plants have an elongate hypocotyl that bears assimilative cotyledons. This type of aboveground germination characterizes hornbeam. In other plants the hypocotyl is underdeveloped, and the cotyledons do not appear on the surface. Such underground germination is typical of the pea and the oak.

The anatomical structure of the hypocotyl retains its embryonic character. It may give rise to adventitious shoots, roots, or leaves (when the apex of the epicotyl is removed, as in cyclamen). In some plants the hypocotyl becomes the repository of nutrients (for example, the tuber in cyclamen). In many root crops, including the turnip, carrot, and sugar beet, the hypocotyl consists of the fleshy part of the storage organ, which is found between the stem base and the root proper.

L. V. KUDRIASHOV