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Thehypocotyl (short for "hypocotyledonous stem",[1] meaning "below seed leaf") is the stem of a germinatingseedling, found below thecotyledons (seed leaves) and above theradicle (root).
As the plantembryo grows at germination, it sends out a shoot called aradicle that becomes the primary root, and then penetrates down into thesoil. After emergence of the radicle, the hypocotyl emerges and lifts the growing tip (usually including the seed coat) above the ground, bearing the embryonic leaves (calledcotyledons), and the plumule that gives rise to the first true leaves. The hypocotyl is the primary organ of extension of the young plant and develops into thestem. The most massive hypocotyl is that ofMyrmecodia lamii which can be 70 cm (28 in) in height by 45 cm (18 in) wide, and is filled with chambers and passageways used by their ant symbionts[2]
The early development of amonocot seedling likecereals and othergrasses is somewhat different. A structure called thecoleoptile, essentially a part of thecotyledon, protects the young stem and plumule as growth pushes them up through the soil. Amesocotyl—that part of the young plant that lies between the seed (which remains buried) and the plumule—extends the shoot up to the soil surface, where secondary roots develop from just beneath the plumule. The primary root from the radicle may then fail to develop further. The mesocotyl is considered to be partly hypocotyl and partly cotyledon (seeseed).
Not all monocots develop like the grasses. Theonion develops in a manner similar to the first sequence described above, the seed coat andendosperm (stored food reserve) pulled upwards as the cotyledon extends. Later, the first true leaf grows from the node between the radicle and the sheath-like cotyledon, breaking through the cotyledon to grow past it.
In some plants, the hypocotyl becomes enlarged as astorage organ. Examples includecyclamen,gloxinia andceleriac. Incyclamen this storage organ is called atuber.
One of the widely used assays in the field of photobiology is the investigation of the effect of changes in light quantity and quality on hypocotyl elongation. It is frequently used to study the growth promoting vs. growth repressing effects of application of plant hormones like ethylene. Under normal light conditions, hypocotyl growth is controlled by a process called photomorphogenesis, while shading the seedlings evokes a rapid transcriptional response which negatively regulates photomorphogenesis and results in increased rates of hypocotyl growth.[3] This rate is highest when plants are kept in darkness mediated by a process called skotomorphogenesis, which contrasts photomorphogenesis.