"kankar". Both rivers have been depressed in the recent past, so that even the highest floods no longer inundate the surrounding plains. In the immediate flood-plains of the rivers there are limited patches of quite modern alluvial deposits, which are subject to redistribution when the streams are in flood. Near the rivers, and especially the Jamna, there are often deep ravines with precipitous sides, formed as the result of rapid erosion during the short monsoon season. During the rest of the year they, are stable.
Except where dissected by these ravines, the surrounding plains are monotonously level. Here and there are slight natural depressions which become shallow lakes during the rainy season, but which are dried up later by evaporation, and by use of the water for irrigation purposes. There are very numerous artificial pools, made for collecting a supply of water to be used for irrigation during the winter and spring. Some of them are deep enough and extensive enough that they retain some water throughout the year. Lastly, there are a very few deeper and more permanent bodies of water created by damming up ravines. Some of the larger natural depressions catch the drainage sufficiently large area to accumulate considerable amounts of salts, chiefly sodium sulphate, mixed with more or less sodium chloride and sodium carbonate, and hence become "alkali", known locally as "usar". Aside from variations in the amount of water present, and from the deposits of usar, the soil of the plains presents a very uniform substratum for the growth of vegetation.
The factors influencing and determining the vegetation.
Climatic factors usually are considered to be the most important factors in the ecology of an area, but in the Upper Gangetic Plain biotic factors are at least of equal importance. The climatic differ only in degree from those met with in other parts of the world. The climate is characterized by striking periodicity. No interpretation of the vegetation can be complete without due consideration of the biotic factors, of which the most important are the very dense agricultural population, and other factors associated with a crowded population.
1. The climatic factors.
The climatic factors that appear to be of greatest importance are rainfall, insolation, temperature, humidity, and air movements. These factors are so distributed as to result in a strongly periodic climate. The data I have made use of are mostly taken from the records of hourly observations at the Allahabad Meteorological Observatory (5), extending over a considerable number of years.
