38 University of California Puhlications . [botany 



which reach up into tlie Souoran Zone, the two floras mixing 

 along the border. 



The presence of the Sonoran ishind lying on the southeast 

 slo])e of the ridge running out from Tahquitz Peak is due to a 

 combination of factors. Besides the effect of slope exposure we 

 have here to consider that of desert winds which, gliding over 

 the ridge of which Antsell Rock is the most prominent feature, 

 strike the slope under consideration, giving to it a warmer and 

 drier climate than it would otherwise have. 



Besides influencing the distribution of plants by its effect 

 upon temperature, slope exposure may also act in a more indu-ect 

 way by altering the moisture conditions. Toward the summit of 

 San Jacinto Mountain are barren south slopes, to which the 

 altitude gives a temperature sufficiently low to support forests in 

 case the other conditions were favorable. This deficiency in 

 trees can be explained in no other way than by the lack of 

 moisture in the soil, since the only tree that could be expected 

 at that altitude is the Murray Pine, which reciuires a moist sub- 

 soil and which is common on north exposures and on benches both 

 above and below the slopes in question. The deficiency in soil 

 moisture is undoubtedly due to the early melting of the snow on 

 these south slopes from which the water quickly escapes, while 

 on north exposures and on the benches the lingering snowdrifts 

 act as reservoirs, and, by their slow melting, supply the soil with 

 the necessary moisture. 



The uniting of slope exposure with other factors to change 

 the position of life zones will be discussed elsewhere. 



EFFECT OF DESERT WINDS. 



It is well known that the air currents which ascend a mountain 

 are usually warm and tend to carry the life zones up, but in 

 considering the air currents of San Jacinto Mountain we nuist 

 distinguish between those coming from the west or southwest 

 and those from the east or northeast, — between ocean breezes 

 and desert winds. It is true that the former have crossed 

 the warm San Jacinto Plains before reaching the mountain, but 

 they are still much less torrid and di^ying than those ascending 

 the east side from the Colorado Desert. 



