798. Buttresses are formed at the junction of the root and trunk, and therefore are part root and part stem. Splendid examples of buttresses are formed on the silk-cotton tree. They are sometimes formed on the elm and other trees in low swampy ground.
799. Fleshy roots, or root tubers.—These are enlargements of the root in the form of tubers, as in the sweet potato, the dahlia, etc. They are storage reservoirs for food. Portions of the roots become thick and fleshy and contain large quantities of sugar, as in the sweet potato, or of inulin (a carbohydrate) in the root tubers of the dahlia and other composites.
800. Water-roots and roots of water plants.—These are roots which are developed in the water, or in the soil. Water-roots are sometimes formed on land plants where the root comes in contact with a body of water, or a stream. Water-roots usually possess no root hairs, or but a few, as can be seen by comparing water-roots with soil-roots, or by comparing roots of plants grown in water cultures. The greater body of water in contact with the root and the more delicate epidermis of the root render less necessary the root hairs. The duck-meats (Lemna) are good examples of plants having only water-roots. Other aquatic plants like the potamogetons, etc., have true roots which grow into the soil and serve to anchor the plant, but they are not developed as special organs of absorption, since the stem and leaves largely perform this function.
801. Holdfasts.—These are organs for anchorage which are not true roots. These are especially well developed in some of the algæ (Fucus, Laminaria, etc.). They are usually called holdfasts. The holdfasts of the larger algæ are mainly for anchoring the plant. They do not function as absorbing organs, and the structure is different from that of true roots.
802. Haustoria or suckers is a name applied to another kind of holdfast employed by parasitic plants. In the dodder the haustorium penetrates the tissue of the host (the plant on which the parasite grows), and besides furnishing a means of attachment, it serves as an absorbing organ by means of which the parasite absorbs food from its host. The parasitic fungi like the powdery mildews which grow on the surface of their hosts have simple haustoria which serve both as organs of attachment and absorption, while in the rusts which grow in the interior of their hosts the haustoria are merely absorbing organs.
803. Rootlets, or rhizoids.—Many of the algæ, liverworts and mosses have slender, hair-like organs of attachment and absorption. These plants do not have true roots. Because of the slender form and small size of these organs, they are called rhizoids, or rootlets. In form many of them resemble the root hairs of higher plants.