Specialized below-ground stems

Potato tubers, iris rhizomes, and tulip bulbs are underground stems that store food for the plant (Figure 8). It sometimes is difficult to distinguish between roots and stems, but one sure way is to look for nodes. Stems have nodes; roots do not.

In potato tubers, for example, the "eyes" are actually the stem's nodes, and each eye contains a cluster of buds. When growing potatoes from seed pieces, it is important that each piece contain at least one eye and be about the size of a golf ball so there will be enough energy for early growth of shoots and roots.

Rhizomes resemble stolons because they grow horizontally from plant to plant. Some rhizomes are compressed and fleshy (e.g., iris), while others are slender and have elongated internodes (e.g., bentgrass). Johnsongrass is an insidious weed principally because of the spreading capability of its rhizomes.

Tulips, lilies, daffodils, and onions produce bulbs, which are shortened, compressed underground stems surrounded by fleshy scales (leaves) that envelop a central bud at the tip of the stem. In November, you can cut a tulip or daffodil bulb in half and see all of the flower parts in miniature.

After a bulb-producing plant flowers, its phloem transports food reserves from its leaves to the bulb's scales. When the bulb begins growing in the spring, it utilizes the stored food. For this reason, it is important not to remove the leaves from daffodils, tulips, and other bulb-producing plants until after they have turned yellow and withered. At that time, they have finished producing the food that will be used for next year's flowering.

There are two types of bulbs: tunicate and nontunicate (Figure 8). Tunicate bulbs (e.g., daffodils, tulips, and onions) have a thin, papery covering, which actually is a modified leaf. It helps protect the bulb from damage during digging and from drying out once it is out of the soil. Nontunicate bulbs (e.g., lilies) do not have this papery covering. They are very susceptible to damage and drying out, so handle them very carefully.

Corms are another kind of below-ground stem. Although both bulbs and corms are composed of stem tissue, they are not the same. Corms are shaped like bulbs, but do not contain fleshy scales. A corm is a solid, swollen stem with dry, scale-like leaves. Gladiolus and crocuses produce corms.

Some plants (e.g., tuberous begonias and cyclamen) produce a modified underground stem called a tuberous stem. These stems are short, flat, and enlarged. Buds and shoots arise from the top (crown), and fibrous roots grow from the bottom.

Other plants (e.g., dahlias and sweet potatoes) produce underground storage organs called tuberous roots. which often are confused with bulbs and tubers. However, these are root tissue, not stem tissue, and have neither nodes nor internodes.