Generally speaking the “Bulb” is used to define many plant forms such as corms, tubers, rhizomes, and tuberous roots. True bulbs contain next year’s plant where as corms, tubers, and rhizomes are actually modified stems. All bulbs should be planted with their tips up.
TRUE BULB
Nearly half of the general listed bulbs in the A-Z guide, are true bulbs. The onion represents the basic structure of this type of bulbous plant. The pear-shaped or oval bulb is a complete plant in miniature. At the heart of the bulb is the embryonic flower surrounded by the undeveloped shoot, and the body of the bulb is made up of a series of fleshy scales. These scales are modified leaves held together by the basal plate at the bottom — the scales contain the nutrients which sustain the plant during the dormant period and the first stages of growth. With most true bulbs the scales are closely packed together but with some types, such as the Lily, they are loose and swollen. Most popular bulbs have a papery skin, the purpose of this tunic is to protect the tissues within. Some true bulbs such as the Lily do not have a tunic and are therefore easily damaged by rough handling. Cardiorinum dies after flowering but other bulbs are perennial. Reproduction is by means of offsets (bulblets).
Examples: Muscari, Tulipa, Narcissus, Lilium, Allium.
CORM
Some corms look like bulbs. Many but not all of these rounded or flattened bulbous plants have a protective smooth or fibrous tunic, and there is the true bulb pattern of a central growing point or two at the top and a basal plate from which the roots arise at the bottom. The structure of a corm, however, is fundamentally different. The nutrient-holding body is a stem base and not a series of scales, and the tunic is made up of the dry leaf bases from the previous season. Another important difference is that a corm lasts just one year. When active growth is underway the food store is depleted and the corm starts to shrivel. At the same time one or more new corms start to develop on top or at the sides of the old one. These new corms form next year’s planting material and will flower during the season. A few corms such as Gladioli form small cormlets around the edge, these tiny corms take 2 to 3 years before they reach the flowering stage.
Examples: Crocus, Gladiolus, Freesia, Ixia, Acidanthera.
TUBER
A tuber is a swollen stem which is born underground like a corm, but the similarity ends there. A tuber does not have a basal plate nor is there a protective fibrous covering. There is no neat organization of the growing points — a potato has the classic arrangement of a tuber. The buds or eyes are scattered over the surface and so stems appear from the sides as well as the top of the structure. There is no standard shape, but they are usually squat and knobby. Most types of tuber get bigger as the plant grows but others diminish in size.
Examples: Cyclamen, Gloriosa, Anemone, Eranthis, Begonia.
RHIZOME
The rhizome, corm and tuber are all bulbous types which are thickened stems filled with nutrients to support the growing plant. Rhizomes differ from the others by growing horizontally and spreading outwards either partly or completely below the soil surface. The main growing point is at the tip of the rhizome, but other buds are formed along the upper surface and along the sides. Roots develop from buds on the underside of the rhizome. Most rhizomes are easy to propagate as the long and branching stem can be cut into segments for planting — make sure each piece has roots and at least one bud.
Examples: Convallaria, Canna, Achimenes, Zantedeschia, Agapanthus.
TUBEROUS ROOT
This bulbous type differs from all the others by being a swollen root rather than a swollen stem or a collection of scale leaves. There are several alternative names — root tuber, tuber-like root, etc. Dahlia is a typical example and is the best known tuberous root. The swollen storage organs are born as a cluster from the crown, which is the base of the old stems. These modified roots provide stored nutrients to the plant — during growth fibrous roots are produced to absorb water and nutrients from the soil. The bulbous plants can be propagated by cutting off individual storage roots with a bud (eye) bearing section at the top.
Examples: Dahlia, Eremurus, Clivia, Alstroemeria, Ranunculus.
PSEUDOBULB
The pseudobulb is a specialized storage organ which is produced by many Orchids. It is a thickened stem base and unlike the other bulbous types on this page, the pseudobulb is both green and above ground. It may be oval, cylindrical, or globular and from it arise both leaves and flower stalk.
Examples: Bletilla, Pleione.