SUCCESSIONS OF THE SAT TAL VALLEY. 243
are characteristic trees of the climax oak forest. They .'require more shaded and moist situations than does the oak, hence in this valley they are found only on north slopes.
The distribution of the oak forest is evidently determined by the degree of protection against dessication. Its moisture requirement is greater than that of the Bauhinia forest for it occurs in depressions, stream valleys, north slopes, and other sheltered aspects. The ridge which forms the watershed to the west of the valley at one place is in- terrupted by a flat shelf which receives the seepage from the higher portions of the ridge. The adjoining portions of the ridge are occupied by Bauhinia forest but this shelf has a small oak grove. The altidudinal distribution of the oak may be a matter of humidity. As I have pointed out the rainfall on the south slopes of the first high range varies but little with altitude. But, because of the lower temperature, the relative humidity, hence the moisture effective for vegetative growth, is greater at the higher altitudes.
Coville (5) has found that in many cases the trees and shrubs of cold climates will not resume growth after dormancy without a certain amount of chilling. The necessity for chilling probably deter- mines why many of the cold region plants, as for example the apple, will not grow to advantage in the tropical regions. One wonders then to what extent temperature itself is a factor in determining the altitudinal range of the hill vegetation.
Broad-leaved sclerophyllous woodlands, according to Schimper (10), are characteristic of warm temperate regions with moist winters and dry summers. Except within the range of a mountain climate they are found only in certain coastal regions. In these regions vegetation is subject to short and irregular periods of rest which may be due either to the cold of winter or to the drought of summer. The winter is not altogether favourable to plant growth because of the low temperature neither is the summer altogether favourable because of the low water supply. By constant retention of leaves the trees of these forests are able to take advantage of the favourable periods in either season.
The climate of the region occupied by this forest in the Himala- yas differs from the climate of a typical sclerophyllous region in the fact that the rainy season does not coincide with the winter. Never- theless the fact remains that there are at all seasons periods which are favourable to plant activity. It would be a loss to the plant to enter these short periods without foliage.
Bulbous and tuberous plants, such as are common in the coastal sclerophyllous regions, are also abundant here. But unlike the former regions this Himalayan formation contains a profusion of
