Page:The Journal of Indian Botany.djvu/347

Rh Such temperatures as the above means alone would not be unfavorable 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 rapidily 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