THE ECOLOGY OF THE UPPEB GANGETIC PLAIN. Si3 



common throughout the year. Various species of Spirogyra are to be 

 found throughout the year. Mougeotia, Hydrodictyon, and Vaucherid 

 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., Woljffia arrhiza Wimm. and Trapa bispinosa Eoxb. 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. Potamoge- 

 ton 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 con- 

 fined 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 ex- 

 posure, 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 Mar* 

 silia sp. There is a number of other species that occur quite 

 oommonly, but are not present in large numbers, and take little part 

 in giving character to the formation. A few characteristictly amphi- 

 bious 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 



