Page:British Flowering Plants.djvu/17

Rh for example) the primary root dies away annually, and is renewed. The aerial roots of Ivy, and of many tropical plants, and the sucking-roots of the Dodder, are adventitious roots.

The tubers of the Potato and the bulbs of Onions

are not true roots, but underground portions of stem.

The stalk, stem, or trunk of a plant rises upward; it bears the leaves and flowers, and is generally more or less branched. The points of attachment of the leaves are called nodes, and the intermediate space is called an internode. The nodes are marked by a thickening of the stem in Knot-grass, and in grasses generally; and in such plants the nodes are often called knots, and the internodes joints. In grasses, the joints are of the considerable length; but in some plants the nodes are so close together that the leaves are crowded into a mass called a "head," as in Lettuce, etc.

The stem may be either simple or branching, woody or herbaceous, and differs much in shape, etc., in different plants (fig. 1, a-i). Thus, the stem may be either cylindrical or angulated, rounded or filiform, erect or decumbent, and so on.