Page:The Journal of Indian Botany.djvu/359

Rh common throughout the year. Various species of Spirogyra are to be found throughout the year. Mougeotia, Hydrodictyon, and Vaucheria are most abundant in later winter and the hot season.

There are a few free-floating aquatic vascular plants, the most important being Ceratophyllum demersum L., Azolla pinnata, Lemna minor L., Wolffia arrhiza Wimm. and Trapa bispinosa Roxb. The first named belongs primarily to the rainy season, the others mainly to the cold season.

Submersed attached aquatic stage. It is in this stage that the vegetation first becomes really abundant, and an important feature in succession. The plants of this stage root in the mud floor of the pool, and grow upward, though always remaining under water. Some of them growing in the rivers are attached 'many feet below the surface, and produce an enormous amount of vegetation. Potamogeton pectinatus L., is able to occupy the deepest water, and is one of the most abundant of the aquatics. Hydrilla verticillata Casp., Naias graminea Delile and Vallisneria spiralis L., are equally abundant, and are found in progressively shallower water. Others very commonly met with are Potamogeton crispus L., Zannichellia palustris L., Chara spp., and Nitella spp. Most of these plants are to be found throughout the year, though they reach their maximum development during the cold season. Chara and Nitella are not so abundant during the winter, because they are plants of shallow water, and are among the first to be destroyed by drying up of the pools.

Attached emersed aquatic stage. Plants of this stage are confined to rather shallow water (Fig. 8), and thus are very liable to be left stranded by lowering of the water level. Some of them are amphibious, and succeed in making fair growth even when exposed for long periods. The vegetative parts of others are killed by the exposure, and only subterannean perennating organs remain alive. The most important and characterestic plants of this stage, approximately in the order in which they appear in the succession, are Nymphaea lotus L., Eleocharis plantaginea Br., Scirpus maritimus L., and Marsilia sp. There is a number of other species that occur quite commonly, but are not present in large numbers, and take little part in giving character to the formation. A few characteristictly amphibious species root in shallow water, and produce extensive floating branches that grow out over the surface of the water (Fig. 9). Chief among these are Ipomaea reptans Poir., Panicum paspaloides Pers., and P. punctatum Burm. They never, however, are permitted to develop the dense floating mats that are so common a feature of aquatic vegetation growing under undisturbed conditions.

Wet meadow stage. This stage comprises the vegetation on the