The hypocotyl (short for "hypocotyledonous stem", meaning "below seed leaf") is the stem of a germinating

seedling A seedling is a young sporophyte developing out of a plant embryo from a seed. Seedling development starts with germination of the seed. A typical young seedling consists of three main parts: the radicle (embryonic root), the hypocotyl (embryo ...

, found below the

cotyledon A cotyledon (; ; ; , gen. (), ) is a significant part of the embryo within the seed of a plant, and is defined as "the embryonic leaf in seed-bearing plants, one or more of which are the first to appear from a germinating seed." The num ...

s (seed leaves) and above the

radicle In botany, the radicle is the first part of a seedling (a growing plant embryo) to emerge from the seed during the process of germination. The radicle is the embryonic root of the plant, and grows downward in the soil (the shoot emerges from ...

( root).

Eudicots

As the plant embryo grows at germination, it sends out a shoot called a

that becomes the primary root, and then penetrates down into the

soil Soil, also commonly referred to as earth or dirt Dirt is an unclean matter, especially when in contact with a person's clothes, skin, or possessions. In such cases, they are said to become dirty. Common types of dirt include: * Debri ...

. 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 (called

s), 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 the stem.

Monocots

The early development of a monocot seedling like cereals and other grasses is somewhat different. A structure called the

coleoptile Coleoptile is the pointed protective sheath covering the emerging shoot in monocotyledons such as grasses in which few leaf primordia and shoot apex of monocot embryo remain enclosed. The coleoptile protects the first leaf as well as the growing ...

, essentially a part of the ''cotyledon'', protects the young stem and plumule as growth pushes them up through the soil. A mesocotyl—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 (see

seed A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiospe ...

). Not all monocots develop like the grasses. The

onion An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus ''Allium''. The shallot is a botanical variety of the onio ...

develops in a manner similar to the first sequence described above, the seed coat and endosperm (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.

Storage organ

In some plants, the hypocotyl becomes enlarged as a storage organ. Examples include cyclamen, gloxinia and celeriac. In cyclamen this storage organ is called a tuber.

Hypocotyl elongation assay

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.Debatosh Da
"Ethylene- and shade-induced hypocotyl elongation share transcriptome patterns and functional regulators"
"Plant Physiology",21-06-2016 This rate is highest when plants are kept in darkness mediated by a process called skotomorphogenesis, which contrasts photomorphogenesis.

References

{{Reflist Plant anatomy Plant morphology Plant reproduction

Eudicots

Monocots

Storage organ

Hypocotyl elongation assay

See also

References