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 (embry ...

, 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 numb ...

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 In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...

Eudicots

As the plant

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

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, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...

. 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 Stem or STEM may refer to: Plant structures * Plant stem, a plant's aboveground axis, made of vascular tissue, off which leaves and flowers hang * Stipe (botany), a stalk to support some other structure * Stipe (mycology), the stem of a mushro ...

Monocots

The early development of a

monocot Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one embryonic leaf, or cotyledon. They constitute one of ...

seedling like

cereal A cereal is any Poaceae, grass cultivated for the edible components of its grain (botanically, a type of fruit called a caryopsis), composed of the endosperm, Cereal germ, germ, and bran. Cereal Grain, grain crops are grown in greater quantit ...

s and other

grasses Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species cultivated in lawns and ...

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 onion ...

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

endosperm The endosperm is a tissue produced inside the seeds of most of the flowering plants following double fertilization. It is triploid (meaning three chromosome sets per nucleus) in most species, which may be auxin-driven. It surrounds the embryo and ...

(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 ''Cyclamen'' ( or ) is a genus of 23 species of perennial flowering plants in the family Primulaceae. ''Cyclamen'' species are native to Europe and the Mediterranean Basin east to the Caucasus and Iran, with one species in Somalia. They gro ...

, gloxinia and

celeriac Celeriac (''Apium graveolens'' var. ''rapaceum''), also called celery root, knob celery, and turnip-rooted celery (although it is not a close relative of the turnip), is a variety of celery cultivated for its edible stem or hypocotyl, and shoo ...

. In

this storage organ is called a

tuber Tubers are a type of enlarged structure used as storage organs for nutrients in some plants. They are used for the plant's perennation (survival of the winter or dry months), to provide energy and nutrients for regrowth during the next growing ...

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