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 cafions, 
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. 
