36 Universiiij of C(difornia Puhlicaiions. [botany 



Ai)plyiiig this formula to the extirme ease, where the rays 

 strike the level at au angle of 45° and where the slope is inclined 

 at an angle of 45° to the level (the rays thus striking the slope 

 perpendicularly), we have the following: 



Heat rec'd by slope at 45° i j.14. 



Heat rec'd by equal area of level land 



In other words, if the sun is 45° above the southern horizon 

 and a south slope is inclined at an angle of 45°, any area on the 

 latter will receive nearly one and one-half times as much heat as 

 will an equal area of level land. 



The other extreme occurs on north slopes which, if inclined 

 at an angle of 45° to the level, will receive no rays whatever as 

 long as the sun is less than 45° above the southern horizon. 



We must not, however, expect to find so marked an effect on 

 the position of the life zones as the above figures would seem to 

 indicate, since air currents and other factors are constantly tend- 

 ing to equalize the temperature of adjoining regions. If the two 

 surfaces differ in character this must also l)e taken into consid- 

 eration, since it would, in most cases, be accompanied by a differ- 

 ence in the coefficients of absorption. It is further to be noted 

 that the ratio between the amount of heat absorbed and the 

 amount reflected increases as the sui-face approaches to the nor- 

 mal of the incident rays, and that the absorbed heat affects 

 primarily the temperature of the soil, while reflected heat affects 

 primarily the temperature of the air. 



Coming now to a consideration of slope exposure as affecting 

 the distribution of plants on San Jacinto Mountain, we may take 

 as examples a few of the more important cases. On making the 

 . ascent from the south side we pass first through a dense chapar- 

 ral formation until, at about 5000 feet altitude, we come out 

 upon the crests of east-and-west ridges, when the whole scene 

 immediately changes, and we descend the cool north slopes be- 

 neath groves of Yellow Pine and Kellogg Oak. This change, 

 which gives us a coniferous forest at even lower altitudes than 

 the higher portions of the chaparral belt, is evidently due to a 

 corresponding change in slope exposure, the chaparral occurring 

 on slopes exposed to the south, the forest on slopes facing the 

 north. 



