THE ECOLOGY OF THE UPPER GANGETIC PLAIN. 301 



Such temperatures as the above means alone would not be un- 

 favorable for plants ; it is the occasional extremes that make plant 

 growth difficult. The mean annual extremes range from 39.6° to 

 114.9° F. On rare occasions the temperature of a winter night may 

 drop down to or below the freezing point, but apparently these 

 exceptional low temperatures are of little importance in determining 

 either the character or the composition of the vegetation, even in 

 depressions where frost is most liable to occur. The highest recorded 

 temperature was 119. 8°F., on June 19, 1878 (8). It is the occasional 

 unusually high temperature during the hot season, together with low 

 humidity and a strong wind, that makes plant life difficult. 



The temperatures given above were taken under open thatched 

 sheds, at a height of 4 feet from the ground. The temperature at 

 the soil surface at times rises very much higher than these figures (6) 

 " The average temperature of the ground surface in India is, at the 

 hottest time of the day in the cold weather from 10° to 20° (F.) 

 above that of the air at 4 feet high. The difference increases until 

 the months of April and May, when the excess is usually as high 

 as 40° and sometimes 45° or 50° (F.) ". Ifc falls rapidly during the 

 rainy season, and is as small in August as in the cold season. 



Humidity. Temperature and humidity seem to be the most 

 important of the climatic factors influencing plant development. 

 Perhaps humidity is the most important, for if the proper balance 

 between water loss and water intake could be maintained, plants 

 would be able to endure without much difficulty the maximum 

 temperatures of the Gangetic Plain. In general, humidity depends 

 on rainfall, but during the cold season the low temperature is an 

 important factor in increasing the relative humidity. 



From a mean of 70.4 per cent, in January, the relative humidity 

 falls rapidly to a minimum of 34.7 per cent, in April, then rises rapid- 

 ily to a maximum of 81.9 per cent, in July. There is a slow fall 

 to 80.2 per cent, in September, then it becomes rapid with the 

 cessation of the rains, down to 68.9 per cent, in October. Here it is 

 overtaken by the falling temperature, and rises to 72.9 per cent, in 

 November, and 70.5 per cent, in December and January. With the 

 rapidly rising temperature and low rainfall of the hot season it falls 

 very rapidly to the April minimum. See Table IV and Figs. 2 and 3. 

 The periods of high relative humidity are optimum for plant growth. 

 As in the case of high temperature, it is the periods of low humidity 

 that cause trouble. The daily humidity range is great during most 

 of the year. Maximum humidity usually occurs at about 6 o'clock 

 and minimum at about 14 o'clock. July shows the least daily range 



2563—39 



