54 THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 



Station 10, almost a pure sand, has a very low wilting coefficient, 

 with no difference of moment between the two depths, since the 

 surface layer has but a very slight admixture of humus. In station 7 

 the effect of surface humus in raising the wilting coefficient is very 

 evident. In station 2 the large percentage of silt and clay is respon- 

 sible for a strikingly high figure. 



Table 8. — Wilting coefficient in stations 10, 7, and 2. 



It has been customary to draw a line to indicate the wilting 

 coefficient upon the graph representing the seasonal march of soil- 

 moisture. Accepting the interpretation of the wilting coefficient 

 presented by Shantz and Moore, we are justified in drawing this 

 line, and in assuming that, when the water-content line falls below 

 that of the wilting coefficient, the vegetation can extract no moisture 

 from the soil. Referring to figure 8, then, we find that in station 

 10 (chaparral) there was at 10 cm. depth no available water from 

 July 1 to November 1. At 100 cm. depth the water-content line, 

 though it is not far above the wilting coefficient, actually touches 

 it but once. In station 7 (forest) conditions at 10 cm. depth were 

 only slightly better than in station 10, water-content and wilting 

 coefficient lines both being relatively and equally high. The period 

 of unavailable water was shorter by the first two weeks of July. 

 At 100 cm. depth conditions were actually less favorable than in 

 station 10, since three times the water-content line descends below 

 that of the wilting coefficient. With regard to the relation between 

 water-content and wilting coefficient, it would seem that there is 

 no striking difference between stations 10 and 7; that soil-moisture 

 conditions, as indicated by these two factors, are about equally 

 severe during the critical period. 



The dry season of 1913 was of unusual severity. That the 

 conditions recorded are extreme is indicated by figure 10, presenting 

 the soil-moisture in relation to the wilting coefficient at the critical 

 period of 1914, which followed a very wet winter. Except for 

 station 10, at 10 cm. depth, the water-content was everywhere 

 well above the wilting coefficient, and the abundance of available 

 water at 100 cm. depth in station 10 is noteworthy. Very likely 

 the average condition is somewhere between the extremes of 1913 

 and 1914, but the special importance of seasons of unusual severity 



