302 



THE JOUENAL OF INDIAN BOTANY. 



and November the greatest, though the range is great throughout the 

 cold season. The daily range during the hot season is intermediate. 



Table IV 



Mean maximum 



Mean 



Mean minimum 



Mean daily range 



88.2 76.5 



70.4 55.6 



43.5 30.9 



I 

 44.7, 45.6 



63.7 

 43.2 

 21.0 



42.7 



53.2 

 34.7 

 18.4 

 34.8 



61.5 70,4 

 41.4 58.6 



25.2 45.4 



36.3 25.0 



89.6:89.3 

 81.9|80.5 

 71.7 70.1 

 17.91 19.2 



91.3 86.4 

 80.2 68.9 

 63.9j43.4 



28.4 43.0 



94.1 88.6 



72.9 

 40.7 

 53.4 



70.5 

 40.5 

 48.1 



Mean, mean maximum and mean minimum relative humidity, and mean 

 daily range, in percent, by months, for Allahabad. 



The mean relative humidity by months gives a very inadequate 

 picture of the severe conditions to which plants are subjected in the 

 hot season. During April, for example, the mean is 34.7 per cent, but 

 the mean minimum attained about 14 o'clock, is 18.4 per cent, and 

 there are many days when the extreme runs much below this ; even at 

 night the mean maximum is only 53.2 per cent. During the rainy 

 season humidity is generally high, and at all times very favorable for 

 plants. During the cold season the mean maximum attains the 

 highest point for the year, reaching 94.1 per cent, in November. 

 Night after night there is a havy fall of dew. At the same time the 

 daily range is greatest during this period, the 14 o'clock mean mini- 

 mum dropping to 40 per cent. . The humid nights permit the 

 vegetation to recover from the drought of daytime. During the 

 hot season, however, there is little opportunity for recovery following 

 the extremely dry day, and none but the most xerophytic of the 

 herbaceous plants are able to survive this trying period. 



Wind. The wind is an important climatic factor in two dis- 

 tinct ways : by bringing in moisture during the rainy season ; and by 

 accentuating the aridity of the hot season. In the latter case only 

 the air currents near the earth are of importance. During most of 

 the year the winds blow fitfully, with a large percentage of the time 

 calm. During March, April and May, wind becomes a very im- 

 portant factor influencing vegetation. Beginning about 11 and con- 

 tinuing till 16 or 17 o'clock, there is a strong wind from the N.N.W., 

 locally known as the " lu." Coming as it does at a time when 

 the relative humidity is at the lowest point, it exerts a powerful 

 dessicating effect on vegetation. The soil is dried out by wind and 

 heat together, and little herbaceous vegetation that is not protected, 

 or favorably situated with regard to water supply, is able to survive. 

 There is little or no air movement at night during the hot season, 



