Rhizomes, Corms and Bulbs.—The typical form of stems is rounded. They are sometimes compressed or flattened laterally, while at other times they are angular. Various terms are applied to the forms of stems, as cylindrical or terete, quadrangular or square, jointed or articu lated, etc. The following are some of the more important modifications of stems : The crown of the root is a shortened stem, often partially underground, which remains in some plants after the leaves, branches and flower-stalks have withered. In this case the internodes are very short, and the nodes are crowded together, so that the plant appears to be stemless. It is seen in perennial plants, the leaves of which die down to the ground annually. A rhizome or rootstock is a horizontal stem usually sending out numerous roots and leaf-buds from its upper surface. It occurs in ferns, iris, Hedychium, Acorus or sweet flag, ginger, waterlily, many species of Carex, rushes, anemone, etc. The leaves are reduced to scales and by their presence, the absence of a root-cap and the presence of a bud at the apex, a rhizome can be distinguished from a root.
A rhizome such as occurs in Solomon's seal is not a single stem, i.e., the product of a single bud, but is composed of portions of successive axes, the aerial parts of which have died off, leaving their scars. A rhizome sometimes assumes an erect form, as in Scabiosa succisa, in which the so-called praemorse root is in reality a rhizome, with the lower end decaying. The erect rhizome of Cicuta virosa (water-hemlock) shows hollow internodes, sepa rated by partitions. In the coral-root or chid Corallorhiza, which grows in soil rich in humus, no roots at all are developed, the coral-like branching rhizome acting as the absorbing organ. A tuber is a thickened stem or branch produced by the reduc tion of the distance between the nodes and the swelling of the internodes, as in the potato. The eyes of the potato are leaf buds. Tubers are sometimes aerial, occu pying the place of branches. The ordinary herbaceous stem of the potato, when cut into slips and planted, sends off branches - from its base, which assume the form of tubers. Tubers frequently store up a quan tity of starch, as in Maranta arundinacea, whence West Indian arrow-root is derived.
Another form of thickened underground stem is the corm, as seen in the autumn crocus (Colchicum), gladiolus, etc. Struc turally it is composed of a solid more or less rounded axis covered by a layer of thin membranous scales. A corm is only of one year's duration, giving off buds annually in the form of young corms. In autumn the young corm gives origin to leaves, the lower of which form sheaths round the corm and flower stalk, the upper remaining very small ; and in the axil of the uppermost leaves the flowering-stem develops and bears the flowers. Meanwhile in the axil of one of the middle leaves on the corm, a bud—the rudiment of a new corm—appears. The flowering-stem dies down, and the young corm from which it arose enlarges greatly during the winter at the expense of its parent corm, which thus becomes shrivelled.
In spring the leaves produced on it, which were merely rudiments in autumn, appear above ground as conspicuous large leaves. At the end of spring these leaves die down, the bases of the lower ones alone remaining, and constituting thin brown scales around the corm. Meanwhile, the young bud corm in the axil of the middle leaf grows rapidly at the expense of its parent corm, but it does not attain a great size. In autumn it produces new leaves, which re main small, but from the axil of the two upper the flowering stem rises up and bears flowers ; whilst in the axil of one of its middle leaves a new bud-corm appears, which will the following autumn produce young leaves, flowering stem, and a new bud-corm, and thus the cycle goes on.
The bulb is another form of underground stem or bud. The axis in this case is much shortened, and the internodes are hardly de veloped. The bases of the leaves rising from the stem are quite close together, and become succulent and enclose the axis. In the lily the thick and narrow scales are arranged separately in rows, and the bulb is called scaly; while in the leek, onion and tulip the scales are broad, and enclose each other in a concentric manner, the outer ones being thin and membranous, and the bulb is tunicated. In the axils of these fleshy scales new lat eral shoots arise, forming new bulbs. The lateral buds sometimes remain attached to the axis, and produce flowering stems, so . that apparently the same bulb continues to flower for many years, as in the hya cinth and tulip; at other times the young bulbs are detached, and form separate plants.
In the axil of the leaves of Lilium bulbiferurn, Dentaria bulbi fera, and some other plants, small conical or rounded bodies are produced, called bulbils or bulblets. They resemble bulbs in their aspect, and consist of a small number of thickened scales enclosing a growing-point. These scales are frequently united closely together, so as to form a solid mass. The scales in bulbs vary in number. In Gagea there is only one scale; in the tulip and Fritillaria imperialis they vary from two to five; while in lilies and hyacinths there are a great number of scales.