A corm, bulbo-tuber, or bulbotuber is a short, vertical, swollen underground
plant stem that serves as a storage organ used by some plants to survive
winter or other adverse conditions such as summer drought and heat.
The word cormous is used to describe plants growing from corms, in analogy to
the use of the terms "tuberous" and "bulbous" to describe plants growing
from tubers and bulbs. Structure
A corm consists of one or more internodes with at least one growing
point, generally with protective leaves modified into skins or tunics. The tunic
of a corm is formed from dead petiole sheaths, 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
as Iridaceae, the tunic of a mature corm can be formidable protection. For
example, some of the larger species of Watsonia accumulate thick, rot-resistant
tunics over a period of some years; the product is a structure of tough,
reticulated fibre. Other species, such as many in the genus Lapeirousia, have
tunics of hard, woody layers. Internally a typical corm mostly
consists of parenchyma cells 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 or stolons on a large corm sprout
independently, forming a tussock. Corms can be dug up and used to
propagate or redistribute the plant. 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 true
bulbs; 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, when a
corm is cut in half it is solid, but when a true bulb is cut in half it is
made up of layers. Corms are structurally plant stems, with nodes and
internodes with buds and produce adventitious roots. On the top of the
corm, one or a few buds grow into shoots that produce normal leaves and flowers.
Cormels Corms can form many small cormlets
called cormels, from the basal areas of the new growing corms, especially when
the main growing point is damaged. They are used to propagate corm-forming
plants. Corms of a number of species of plants are replaced every year by the
plant with growth of a new corm; this process starts after the shoot has
developed fully expanded leaves. The new corm forms at the shoot base just above
the old corm. As the new corm is growing, short stolons are produced that
end with the newly growing small cormels. As the plants grow and flower,
the old corm is used up and shrivels away. The new corm that replaces the old
corm grows in size, especially after flowering is done.
The old corm produces the greatest number of cormels when it is close to
the soil surface. The 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 countries where geophytes are common, so
are animals that feed on them, whether from above like pigs, or from below like
bulb weevils, mole rats, or pocket gophers. Such animals eat through
protective tunics, but they generally will miss several cormels that remain in
the soil to replace the plant consumed. Plants such as Homeria, Watsonia and
Gladiolus, 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. Homeria species produce bunches of cormels on underground stem nodes,
and Watsonia meriana for example actually produces cormels profusely from
under the bracts on the inflorescences. Roots
Many corms produce two different types of roots. Those growing from the bottom
of the corm are normal fibrous roots, they are formed as the shoots grow, and
are produced from the basal area at the bottom of the corm. The second type of
roots are thicker layered roots that form as the new corms are growing, they
are called contractile roots and 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 however, contractile roots seem to be a defence
against digging animals and can bury the corm surprisingly deeply over the years.
Wurmbea marginata is one example of a small plant that can be challenging to
dig unharmed out of a hard, clayey hillside.
Cormous plants Cultivated plants that form corms
include: Alismataceae
Sagittaria spp. Araceae
Arisaema Taro
Xanthosoma spp. Konjac
Pulaka Asparagaceae
Bessera Brodiaea
Dichelostemma Milla
Tecophilaea Asteraceae
Liatris Colchicaceae
Colchicum Cyperaceae
Eleocharis dulcis Iridaceae
Crocosmia Crocuses, including the saffron crocus
Dierama Freesia
Gladiolus Some species of irises
Romulea Musaceae
Bananas Ensete spp.
See also Rhizome
Tuber Root vegetable
References