104 VEGETATION OF A DESERT MOUNTAIN RANGE. 



out by Hall,* who shows that the amount of radiant energy reaching 

 a south slope of 45°, with the sun 45° above the horizon, is 1.4 times 

 as great as that reaching a level piece of ground through an aperture 

 of the same size. It is through this difference, which is still greater 

 between south and north slopes, that the soil is given a higher tempera- 

 ture, that the air is warmed to a higher degree through radiation, and 

 the soil dried more rapidly on all south-facing exposures. 



THE ROLE OF STREAMS AND FLOOD-PLAINS. 



In the description of the vegetation of the Santa Catalinas constant 

 allusion has been made to the distinctive plant communities of springs, 

 streams, flood-plains, and arroyos. The contrast between the vege- 

 tation of these moist or relatively moist situations and that of the 

 mountain slopes is very striking at the mouths of the larger canons, 

 and throughout the Desert and Encinal regions. At the higher alti- 

 tudes, and particularly in the Fir Forest, the moist habitats are not 

 only less striking to the casual observer, but their vegetation actually 

 comprises a great many species which are frequently found away from 

 proximity to streams. 



The influence of streams and flood-plains consists, in brief, in bring- 

 ing components of the upland vegetation of each altitude down along 

 the streamways of the altitudes just below. In this manner the Encinal 

 is traversed by bands of Forest, and the Desert slopes are traversed 

 by bands of Encinal. Furthermore, the streams and springs of the 

 mountain afford the sole habitats for a number of species of aquatic 

 and palustrine plants which do not appear on the upland at any 

 elevation. 



The mechanical agencies of gravity, sheet-floods, and stream flow 

 are all capable of aiding in the downward dissemination of the seeds 

 of all mountain plants and these mechanical agencies should assure 

 the occurrence of all mountain plants in all situations at lower altitudes 

 in which they are capable of survival. The number of seeds which 

 are borne down by streams is, of course, enormous, and the number of 

 resulting germinations is probably very large. The number of sur- 

 vivals, however, is controlled by the physical conditions of the new 

 low-altitude habitat, and in a manner to be further considered. 



In the discussion of slope exposure no account has been taken of the 

 occurrence of plants along streamways at elevations below their lowest 

 upland occurrence, since the individuals along the arroyos and streams 

 are subjected to a very different set of environic controls from those 

 that determine the location of the upland individuals. In the earlier 

 discussion of the vertical limits of species the streamway occurrences 

 were taken into account. 



* Hall, H. M. A Botanical Survey of San Jacinto Mountain. Univ. Cal. Pubn. Bot., vol. i, 

 pp. 1-140, 1902. 



