42 University of California Publications. [botany 



tiire during the day, while throughout the night time the cohl 

 air of the higher altitudes settles into the canons through which 

 they flow. As a result we find the life zones dij^ping along the 

 streams and sometimes carried several hundred feet below their 

 normal position. It is to be noted, however, that while this 

 effect is nearly always one which may be easily determined, it is 

 scarcely to be compared with that produced by some of the 

 factors mentioned above, some of which are capable of shifting 

 the life zones for a vertical distance of several thousand feet. 



THE BATTLE OP THE FACTORS. 



Having once determined the factors which affect the distribu- 

 tion of plants in any region, and having received some notion of 

 the relative value of each of these, the next problem is the 

 determination of the result produced by these factors when act- 

 ing in all possible combinations. In attacking a phytogeo- 

 graphic problem there is a strong tendency to select only one or 

 two factors and, centering the attention on these, to attempt the 

 explanation of all phenomena connected with the subject by 

 reference to these alone ; but it is to be remembered that the flora 

 of any region is determined by a number of factors, some of 

 greater importance and some of less, but each to he considered 

 as having its own influence on the common resultant. 



A number of these factors often act to produce the same 

 result, and whenever this occurs a marked effect upon the 

 , position of the floral belts is noticed. An instance of this is 

 found along the North Fork of the San Jacinto River, where the 

 effects of slope exposure and of the presence of a mountain 

 stream act in unison against that of altitude. Here, as a result 

 of the cooling effects produced by the stream and by the 

 presence of Indian Hill which, rising on the west for some 2000 

 feet above the stream banks, shyts off the afternoon sun, we find 

 the lower edge of the Yellow Pine belt dipping to an altitude of 

 3000 feet, while its normal position on San Jacinto Mountain is 

 about 2000 feet higher. 



A fierce battle of this nature is the one waged on the north 

 side of the mountain where, on account of the steep north 

 slopes, we should expect to see the life zones running down to 



