Page:The Journal of Indian Botany.djvu/351

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. Mean temperature, mean sunshine (mean cloudiness curve inverted), 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.