Botany for Gardeners - Going Underground

For most plants, growth begins on or in the soil when the seed germinates, and then life continues with the production of stems, leaves, flowers and fruit. Life underground goes on, though, and even expands and diversifies in the cases of plants that live from year to year. The stem, which is the support structure for the leaves, flowers and fruits, is also the conduit through which water and minerals flow from the roots to the rest of the plant, and through which carbohydrates move from the leaves to the roots. Stems branch above ground, but they also grow in various ways below ground, as I will describe shortly.

Plant roots are the structure that enables a plant to take up water and minerals from the soil. They may be fibrous, coarse, branched, or with a main or central part called a tap root. Unlike stems, true roots do not have buds, leaves, or other parts that correspond to the above-ground plant parts. So what, then, are those things that look like roots but that have a baby plant on the end of them?

Stems go underground

Various types of plants can produce a variety of modified stems that grow along the ground or underground rather than up in the air. Chief among them are the stolon, the rhizome, the bulb, the corm and the tuber. Each of these is a stem, but modified so as to serve a different function for the plant. Stolons and rhizomes function primarily as propagative structures, although some, as in Iris (see thumbnail picture above, right), also serve as storage organs. Stolons are stems that grow along the surface of the soil and produce little plants at their ends. They also produce leaves at joints along the stolon. These joints are called nodes. Rhizomes are similar but grow beneath the soil, and may produce plants along their length at nodes that correspond to the joints where leaves are attached on above-ground stems. The presence of nodes is also confirmation that the stolon or rhizome is not a root because roots have no nodes. Even when a root is capable of producing a plant, as in sweet potato, the bud that yields the plant is not at a node, but is a spontaneously formed bud produced from tissue in the root. These spontaneous buds are known as adventitious buds, while the kind of bud found at a node is known as an axillary bud. When swollen, these roots are known as tuberous roots (see picture above, left).

The bulb is composed of a very short, thick and flattened stem that has fleshy leaves attached to it, overlapping and forming what appear to be layers, as in the onion or tulip bulb (see picture at right). These fleshy leaves are where the food is stored for the plant to use in next season's growth spurt. Similar to the bulb in shape is the corm, in which the stem part is much thicker and much enlarged when compared to the stem in a bulb, and the fleshy leaves are reduced to dry scaly structures. Gladiolus is an example of a plant with a corm. In corms; the interior is solid and the scaly leaves dry and thin on the exterior (see picture below, left).

Corms can be produced at the ends of rhizomes, too, as in some Alocasia species. This type of corm is similar to a tuber except that the corm has the dry scaly leaves on the outside while the tuber has none.

Some say "potato" and some say . . . tuber?