Bulbs, Corms and Tubers
Bulbs, corms and tubers are organs made from swollen plant parts, modified to store food.
These structures grow from food created during the growing season.
The stored food is used to feed the plant when it starts growing again after a period of dormancy.
There are two types of true bulbs, tunicate and scally bulbs.
The onion is typical of a tunicate bulb.
This bulb is made up of concentric layers fleshy leaves, held together by the basal plate.
The basal plate is a small crown that contains one or more shoots for the next season's growth.
It is also where roots are formed.
The tunic or skin is made up of old leaf bases.
It not only protects the bulb from mechanical damage, but prevents it from losing water while dormant.
Bulbs that form new roots every year (for example,
snowflakes) should be stored dry, but those that have roots that persist from year to year, such as
Hippeastrum, should be stored in saw dust or wood shavings.
Care must be taken when planting and storing these bulbs that the roots are not damaged.
Lily is an example of a scally bulb.
In these bulbs, circular rows of swollen scales, packed loosely, store food for the next season's growth.
As these bulbs lack a tunic, they can be easily damaged while being handled, and must not be stored completely dry.
Certain bulbs use up all their food reserve to flower and grow and recreates a new bulb from the food stored from the current season's growth.
Examples are several specias of Allium and
Others only use up fraction of the reserves, and the bulb gets bigger every year, for example,
Propagation is either by seed or by detaching small bulbs (bulblets) produced on the side of the mother bulb.
Certain species produce bulblets on the flowering stem or all along leaf axils of the stem (bulbil), for example, certain Lillium species.
Propagation is also possible from detached scales and bulb pieces.
A corm looks and functions like a bulb but is fundamentally different from a bulb.
It is made up of a swolen stem base of a shoot, wrapped with tunic from old leaf bases.
There can be one of more growth eyes on the top of the corm.
When growth starts, the old corm withers away and a new corm is produced in its place, on top of the old corm.
Some corms produce numerous little corms (cormel) at the base of the new corm.
These will germinate and grow into new plants, but may take several years to flower.
Certain corms, for example
Freesia, will produce corms along leaf axils of the flowering stem.
Other example of corm forming plants are
Sparaxis and Gladiolus.
Most corms should be stored completely dry or they will rot.
Some corms, such as the Water Chestnut, should be stored moist.
A tuber is a swollen stem or crown.
One or more growth eyes on the tuber will sprout to produce next season's growth.
When growth starts, some tubers wither away (for example,
Sandersonia), but others keep growing bigger (for example,
Roots often form on the underside of the tuber, but may also grow from the top.
For example, in
Cyclamen, the roots only form on the sides and bottom of the tuber, but in
Begonia, roots form on all sides.
Some tubers do not have roots at all, but instead the roots form on the current season's growth.
Tubers are usually underground, but on some species, tubers may be formed above ground.
When these tubers come in contact with the soil, they will usually root.
An example is Ceropegia woodii.
This creeping plant form tubers along leaf axils on its stem.
A Root tuber is a swollen root.
Certain species have growth eyes on the root tuber, for example kumara or sweet potato.
Next season's growth will sprout from these eyes.
Some root tubers do not have any eyes and will not sprout if detached from the crown.
Dahlia and Rununculus.
Next season's growth will only sprout from eyes on the crown.
The tubers are purely for food storage and have no reprodutive use.
Contrary to popular belief, a potato is not a root tuber, but rather a tuber made from swollen stolon (runner).
A rhizome is a swolen horizontal stem that spreads as it grows.
Growth is from the tip of the rhizome and buds that form along its length.
Adventitious roots form where the rhizome touches the ground.
Rhizomes may either be underground, for example plants in the ginger family and
Zantedeschia, or above ground as in
Iris and certain species of ferns.
Orchids grow varying length of rhizomes that connect their pseudobulbs.
Certain species lack pseudobulbs and grow leaves directly from the rhizomes.
A pseudobulb is a special organ found in orchids that is actually a swollen stem or stem base.
Pseudobulbs come in all shapes and sizes, and some only lasts one season (for example Pleione and Calanthe), while others persists for years, even after the leaves have fallen off (for example,
Some are attached to creeping rhizomes and form clumps.
Pseudobulbs from epiphytic (grows on trees) and lithophytic (grows on rocks) orchids are above ground and typically form new shoots from the base.
Pseudobulbs from terestrial orchids are below ground and function much like a tuber (for example,
All are created by the current season's growth and store food to start off the next season's growth and flowers.
Propagation is usually by detaching back bulbs (old pseudobulbs that have lost their leaves), as in
Cymbidium, splitthig clumps by cutting across the rhizome (for example, Cattleya), or through kei keis (shoots that form along the length or top of the pseudobulb or stem).
Not all orchids form pseudobulbs.