SUCCESSIONS OF THE SAT TAL VALLEY. 241
only on the sunny faces of the Sat Tal valley, south, east, and west, and is with the exception of a few scattering outliers entirely absent from the higher valleys. Its upper limit is roughly 4000 feet, on north facing slopes and 5500 feet on south facing slopes.
The factors which induce the shedding of the old leaves and the development of the new deserve careful investigation. The end of May, 1921 was an unusually dry time in the Western Himalayas. At Sat Tal the rainfall for the eight dry season months ■ October, 1920 to May, 1921 inclusive, was 6*29 inches and was limited to two of these months. The rainfall for the corresponding period 1918-19 was 17'17 inches and was distributed through seven of the eight months. A com- parison of notes on the vegetation in May of the two years shows that in almost all Species of the deciduous forest the shedding of the old leaves occurred earlier in 1921 than in 1919. Hence it seems appa- rent that the shedding of the foliage is hastened by lack of humidity. On the other hand observations show that the putting out of new foli- age by no means awaits the beginning of the monsoon rains. Schim- per (10) says that the trees of monsoon districts renew their foliage at or immediately before the commencement of monsoon rains, but he throws no light on the nature of the stimulus which causes leafing in anticipation of rain. The chief surprise that the writer has met in his observation of the monsoon forest on the plains of India is the fact that many, indeed most, species go through the exceedingly severe heat and drought of May and June with young tender foliage. At Sat Tal there were no showers in May, 1921 to indicate the coming of the monsoon, yet the leafing of the monsoon trees had proceeded further than in 1919 when such showers did occur. The species latest to put forth their foliage are Nyctanthes arbortristis, Grewia tiliaefolia&nd Euphorbia Boyleana, yet in numbers of Grewia trees and in one Euphorbia leaves were coming out before the end of May. Instead of being a benefit to the tree, preparing it to profit from the rains without loss of time, this early leafing seems in some cases a real detriment, for the drying of the tender foliage has gone on to such an extent as to threaten the life of trees and shrubs of several species.
It may be that the development of leaf buds requires a certain period of incubation. In some cases greater insolation seems to induce earlier leafing. Temperature-controlled experiments or ob* servations checked with a thermograph are most desirable to test this point. The way in which temperature most probably acts upon leafing is by hastening the maturation of the fruit. In at least twenty of the monsoon forest species, notable among which are the tree Bauhinias, Erythrina suberosa, Oligemia dalbergioides, Odina Wodier,
