Of these stages the first is quite commonly observed in pools and streams; the second is not well represented; the third is the present modified climatic climax; stage four is not found well developed anywhere, but fragments of it are to be seen in protected places, and in the thorny shrubs and small trees scattered over certain areas; stages five and six are largely hypothetical, and six would be the true climatic climax for this portion of the Gangetic Plain.
Aquatic stage. As has been said, the aquatic stage is to be found in the numerous pools that are scattered everywhere over the area. Most of these pools are very small and shallow (Fig. 9); a few may cover an area of several acres; some dry up after the close of the rains, while others retain their water up into the cold season, and a few are permanent. The smaller and shallower and less permanent pools show only the earlier phases of the aquatic stage, for the natural vegetation is always interfered with by man. The constant use of a pool for various domestic purposes destroys most or all of the plants, and irrigation finally removes the water and leaves a mud flat that quickly dries out without being able to support much in the way of plant growth. The green plants in proximity to water are eaten off by grazing animals. Shallow pools are planted with rice during the rains (Fig. 9), and this effectively prevents development of the characteristic wild aquatic vegetation. The small number of native plants that can grow under such conditions are more of the nature of ruderals. Even if a pool is relatively free from human disturbance, the water level is rapidly lowered by evaporation, so that plants that are strictly aquatic during the rains are finally stranded on mud flats to dry out and die, or to survive by means of perennating organs till inundated during the next rainy season (Fig. 8.).
The aquatic stage may be further divided into:—
- Free-floating aquatic stage;
- Attached submersed aquatic stage;
- Attached emersed aquatic stage.
These stages overlap each other more or less, but they indicate the actual order of normal succession.
Free-floating aquatic stage. This stage is represented mainly by algae, of which there is great variety and considerable abundance. The principal algae are several species of Cyanophyceae, Spirogyra spp., and Cladophora spp; others very commonly found, but abundant only in very limited areas are Mougeotia spp., Oedogonium spp., Hydrodictyon and Vaucheria. The Cyanophyceae are specially abundant in the shallow rainy season pools, though they are rather
