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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.
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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,
