SUCCESSIONS OF THE SAT TAL VALLEY. 247
difference here is probably purely a matter of evaporation. The south slope, being exposed to the sun more directly and for longer periods, will not retain moisture enough to permit the oak forest to become established.
A precipitious slope will in general not allow trees to get a foot- hold, with the exception of special scrambling forms such as Ficus foveolata. With time such slopes become more gentle and support a more luxuriant forest vegetation.
An exposed rock is occupied by lichens. These are followed by xerophytic mosses, herbs, grasses and sedges and finally by shrubs and trees, the trees passing from xerophytic to mesophytic.
A typical xerarch succession then would be lichens, mosses, grass- lands, Euphorbia, pine, oak. Instead of the pine there might be substituted, following Euphorbia, such semi-shrubby pioneers as Woodfordia, Flacourtia and Nyctanthcs, then the Bauhinia formation, finally the oak forest.
In parts of this valley and the neighboring region are steep slopes of shale or other soft rock which readily crumbles giving rise to landslips with well-developed talus slopes. Both the exposed rock face and the talus have contours so temporary that their occupation by plants may be prevented for years. "When finally the contour be- comes more permanently established the first plants to appear are generally xerophytic grasses and sedges with an admixture of ruderal herbs, such as Rumex hastatus and the American Tridax procumbens and Oenothera rosea. These are followed by shrubs, among which the Berber is-Rosaceae group is often prominent, and the shrubby vegeta- tion very gradually gives way to the climatic forest. If shrubs and trees which have the power of producing adventitious buds from the roots have grown previously on the slope these may contribute largely to the new population by suckering from the exposed roots on the denuded area or by the continued growth of the slumping vegeta- tion on the talus slope.
Euphorbia royleana, the most extreme xerophyte among the trees of this region, is admirably fitted for the place it fills. Its fleshy angled cactus-like stems hold large quantities of water and do the greater part of the synthetic work, the leaves being very transient.
The shrubby formation of Woodfordia, Nyctanthes and Flacourtia occupies considerable areas on the south-facing slope at the north end of the valley. It is for the most part a relatively dense growth, difficult of penetration, but the individual members are low, usually not over twice the height of a man. Woodfordia is a pioneer which is often met with on bluffs and even in stream valleys in the plains as well as in the lower hill country.
