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. 



