THE ECOLOGY OF THE UPPER GANGETIC PLAIN 297 



1 kankar". Both rivers have been depressed in the recent past, so 

 that even the highest floods no longer inundate the surrounding plains, 

 [n 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 depres- 

 sions 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 depres- 

 sions catch the drainage sufficiently large area to accumulate consider- 

 able 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. 



