The _Siphoneæ_ are algæ occurring both in fresh and salt water, and are distinguished from other algæ by having the form of a tube, undivided by partition walls, except when reproduction occurs. The only common representatives of the order in fresh water are those belonging to the genus _Vaucheria_, but these are to be had almost everywhere. They usually occur in shallow ditches and ponds, growing on the bottom, or not infrequently becoming free, and floating where the water is deeper. They form large, dark green, felted masses, and are sometimes known as "green felts." Some species grow also on the wet ground about springs. An examination of one of the masses shows it to be made up of closely matted, hair-like threads, each of which is an individual plant. In transferring the plants to the slide for microscopic examination, they must be handled very carefully, as they are very easily injured. Each thread is a long tube, branching sometimes, but not divided into cells as in _Spirogyra_ or _Cladophora_. If we follow it to the tip, the contents here will be found to be denser, this being the growing point. By careful focusing it is easy to show that the protoplasm is confined to a thin layer lining the wall, the central cavity of the tube being filled with cell sap. In the protoplasm are numerous elongated chloroplasts (_cl._). and a larger or smaller number of small, shining, globular bodies (_ol._). These latter are drops of oil, and, when the filaments are injured, sometimes run together, and form drops of large size. No nucleus can be seen in the living plant, but by treatment with chromic acid and staining, numerous very small nuclei may be demonstrated. [Illustration: FIG. 21.--_A_, _C_, successive stages in the development of the sexual organs of a green felt (_Vaucheria_). _an._ antheridium. _og._ oögonium. _D_, a ripe oögonium. _E_, the same after it has opened. _o_, the egg cell. _F_, a ripe spore. _G_, a species in which the sexual organs are borne separately on the main filament. _A_, _F_, × 150. _G_, × 50. _cl._ chloroplasts. _ol._ oil.] When the filaments are growing upon the ground, or at the bottom of shallow water, the lower end is colorless, and forms a more or less branching root-like structure, fastening it to the earth. These rootlets, like the rest of the filament, are undivided by walls. One of the commonest and at the same time most characteristic species is _Vaucheria racemosa_ (Fig. 21, _A_, _F_). The plant multiplies non-sexually by branches pinched off by a constriction at the point where they join the main filament, or by the filament itself becoming constricted and separating into several parts, each one constituting a new individual. The sexual organs are formed on special branches, and their arrangement is such as to make the species instantly recognizable.