=722. Floating stems.=—These are necessarily found in aquatic plants. The stems may be ascending or horizontal. The stems are usually not very large, nor very strong, since the water bears them up. The plants may grow in shallow water, or in water 10-12 feet or more deep, but the leaves are usually formed at or near the surface of the water in order to bring them near the light. Various species of Potamogeton, Myriophyllum, and other plants common along the shores of lakes, in ponds, sluggish streams, etc., are examples. Among the algæ are examples like Chara, Nitella, etc., in fresh water; Sargassum, Macrocystis, etc., in the ocean. In these plants, however, the plant body is a thallus, which is divided into stem-like (_caulidium_) and leaf-like (_phyllidium_) structures. =723. The burrowing type, or rhizomes.=—These are horizontal, subterranean stems. The bracken fern, sensitive fern, the mandrake (see fig. 413_a_), Solomon’s seal, Trillium, Dentaria, and the like, are examples. The subterranean habit affords them protection from the cold, the wind, and from injury by certain animals. Many of these stems act as reservoirs for the storage of food material to be used in the rapid growth of the short-lived aerial shoot. In the ferns mentioned, the subterranean is the only shoot, and this bears scale leaves which are devoid of chlorophyll, and foliage leaves which are larger, and the only member of the plant body which is aerial. The foliage leaf has assumed the function of the aerial shoot. The latter is not necessary since flowers are not formed. The mandrake, Solomon’s seal, Trillium, etc., have scale leaves on the fleshy underground stems, while foliage leaves are formed on the aerial stems, the latter also bearing the flowers. Some of the advantages of the rhizomes are protection from injury, food storage for the rapid development of the aerial shoot, and propagation. Many of the grasses have subterranean stems which ramify for great distances and form a dense turf. For the display of foliage and for flower and seed production, aerial shoots are developed from these lateral upright branches. III. Specialized Shoots and Shoots for Storage of Food.[40]