THE ECOLOGY OF THE UPPER GANGETIC PLAIN. 305 



The hot season is ushered in with the rising temperature and 

 decreasing humidity of spring, and extends from the first of March 

 to the middle of June, It is characterized by low rainfall, high 

 insolation, high temperature, low humidity, and strong winds. April 

 and May are typical of this season. The April temperature ranges 

 from 73.4° to 102.6° P., and the humidity from 53.2 to 18.4 per cent. 

 (Table V, and Figs. 6 and 7). Both temperature and humidity are 



Fig. 7 



Fig. 7. Mean temperature, mean sunshine (mean cloudiness curve in- 

 verted), mean rainfall, and mean relative humidity for the year 

 at Allahabad, so calculated that all the maxima touch the top of 

 the graph and the minima touch the bottom. This emphasizes 

 the three climatic seasons, which determine three corresponding 

 vegetational seasons. 



slightly higher in May. The climatic conditions are distinctly 

 xerophytic, with the result that the mesophytic vegetation of the 

 cold season disappears, and only those plants that are adapted to 

 withstand the severe drought can survive. There are limited local 

 areas that for some reason or other are able to supply sufficient 

 moisture to sustain mesophytic plants, and it is in such places 

 only that the vegetation remains distinctly green. 



