Corm

For other uses, seeCorm (disambiguation).

Not to be confused withCorn.

Tarocorms for sale in aRéunionmarket

Corm,bulbo-tuber,orbulbotuberis a short, vertical, swollen undergroundplant stemthat serves as astorage organthat some plants use to survive winter or other adverse conditions such as summer drought and heat (perennation).

The wordcormoususually means plants that grow from corms, parallel to the termstuberousandbulbousto describe plants growing fromtubersandbulbs.

Crocosmiacorm with the tunic partly stripped to show its origin at the nodes on the cormcortex.

Crocosmiacorm anatomy, showing tunic,cortexof storage tissue, central medulla, and emergence of a new corm from a bud near the top.

Crocosmiacorm withstolonsemerging through the tunic. The stolons originate at the axillary buds of the corm scales, and generally produce new corms at their tips

A corm consists of one or moreinternodeswith at least one growing point, generally with protective leaves modified into skins ortunics.The tunic of a corm forms from deadpetiolesheaths—remnants of leaves produced in previous years. They act as a covering, protecting the corm from insects, digging animals, flooding, and water loss. The tunics of some species are thin, dry, and papery, at least in young plants, however, in some families, such asIridaceae,the tunic of a mature corm can be formidable protection. For example, some of the larger species ofWatsoniaaccumulate thick, rot-resistant tunics over a period of years, producing a structure of tough, reticulated fibre. Other species, such as many in the genusLapeirousia,have tunics of hard, woody layers.

Internally, a typical corm mostly consists ofparenchymacells, rich in starch, above a circular basal node from which roots grow.

Long-lived cormous plants vary in their long-term development. Some regularly replace their older corms with a stack of younger corms, increased more or less seasonally. By splitting such a stack before the older corm generations wither too badly, the horticulturist can exploit the individual corms for propagation. Other species seldom do anything of that kind; their corms simply grow larger in most seasons. Yet others split when multiple buds orstolonson a large corm sprout independently, forming a tussock.

Corms can be dug up and used to propagate or redistribute the plant (see, for example,taro). Plants with corms generally can be propagated by cutting the corms into sections and replanting. Suitably treated, each section with at least one bud usually can generate a new corm.

Comparison to bulbs

Corms are sometimes confused with truebulbs;they are often similar in appearance to bulbs externally, and thus erroneously called bulbs. Corms are stems that are internally structured with solid tissues, which distinguishes them from bulbs, which are mostly made up of layered fleshy scales that are modified leaves. As a result, a corm cut in half appears solid inside, but a true bulb cut in half reveals that it is made up of layers.Corms are structurally plant stems, with nodes and internodes with buds and produceadventitiousroots. On the top of the corm, one or a few buds grow into shoots that produce normal leaves and flowers.

Gladioluscorm, showing the formation of small cormels at the ends of short stolons

Cormels

Corms can form many small cormlets calledcormels,from the basal areas of the new growing corms, especially when the main growing point is damaged. These propagate corm-forming plants. A number of species replace corms every year by growing a new corm. This process starts after the shoot develops fully expanded leaves. The new corm forms at the shoot base just above the old corm. As the new corm grows, shortstolonsappear that end with the newly growing small cormels. As the plants grow and flower, they use up the old corm, which shrivels away. The new corm that replaces the old corm grows in size, especially after flowering ends.

The old corm produces the greatest number of cormels when close to the soil surface. Small cormels normally take one or two more years of growth before they are large enough to flower.

Cormels do have a reproductive function, but in the wild they also are important as a survival strategy. In most places wheregeophytesare common, so are animals that feed on them, whether from above like pigs, or from below like bulb weevils,mole rats,orpocket gophers.Such animals eat through protective tunics, but they generally miss several cormels that remain in the soil to replace the consumed plant. Plants such asHomeria,WatsoniaandGladiolus,genera that are vulnerable to such animals, are probably the ones that produce cormels in the greatest numbers and most widely distributed over the plant.Homeriaspecies produce bunches of cormels on underground stem nodes, andWatsonia merianafor example actually produces cormels profusely from under bractson theinflorescences.

Those growing from the bottom of the corm are normal fibrous roots formed as the shoots grow, and are produced from the basal area at the bottom of the corm. The second type are thicker layered roots calledcontractile rootsthat form as the new corms are growing. They pull the corm deeper into the soil. In some species contractile roots are produced in response to fluctuating soil temperatures and light levels. In such species, once the corm is deep enough within the soil where the temperature is more uniform and there is no light, the contractile roots no longer grow and the corm is no longer pulled deeper into the soil. In some other species, contractile roots seem to be a defence against digging animals and can bury the corm surprisingly deeply over the years.Wurmbea marginatais one example of a small plant that can be challenging to dig unharmed out of a hard, clayey hillside.

Corms plants

Corm onAlocasia macrorrhizos

Cultivated plants that form corms include:

• Alismataceae
  • Sagittariaspp. (arrowhead or wapatoo)
• Araceae
  • Alocasia macrorrhizos(giant taro)
  • Arisaema
  • Colocasia esculenta(taro)
  • Cyrtosperma merkusii(giant swamp taro)
  • Xanthosomaspp. (malanga, cocoyam, tannia, and other names)
• Asparagaceae
  • Bessera
  • Brodiaea
  • Dichelostemma
  • Milla
  • Tecophilaea
• Asteraceae
  • Liatris
• Colchicaceae
  • Colchicum
• Cyperaceae
  • Eleocharis dulcis(Chinese water chestnut)
• Iridaceae
  • Crocosmia(Montbretia)
  • Crocuses,including thesaffroncrocus (Crocusspp.)
  • Dierama
  • Freesia
  • Gladiolus
  • Some species ofirises(Irisspp.)
  • Romulea
• Musaceae
  • Bananas(Musaspp.)
  • Ensetespp. (enset)

See also

• Rhizome
• Root vegetable
• Tuber