THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 53 



A second series of determinations was made at a later time. The 

 percentages are somewhat higher, but the relative values are very 

 similar in most cases, which is the important thing. The series 

 given here is the more valuable, since the soils used were composites 

 of many individual samples. 



Wilting coefficient. — The method developed by Briggs and Shantz 

 (15) was followed, and the soils to be tested were brought from Jasper 

 Ridge to the University of Minnesota. The experiment was carried 

 on in the university greenhouse, in a room where the relative 

 humidity averages 30 to 40 per cent during the middle of the day. 

 An atmometer was kept in operation close to the jars during most 

 of the period of growth and wilting. The average daily evaporation 

 recorded, reduced to the usual standard, was 10.2 c. c. The extremes 

 were 14.7 c. c. and 3.1 c. c. Glass tumblers were used for containers 

 and a constant soil temperature was maintained by immersing them 

 almost to the top in slowly running water. Kubanka wheat, 

 obtained through the kindness of Dr. Briggs and Dr. Shantz, was 

 used as the indicator plant. 



The constant termed the "wilting coefficient" has been variously 

 interpreted. The latest and most reasonable conception, due to 

 Shull (82) and Moore (65), is that it is purely a function of the 

 soil, "a point in the water-content of the soil at which the water 

 practically ceases to move along the film (on the soil particle), no 

 matter how sharp the gradient at the edge of the film," and not a 

 matter of "balance between the 'back-pull' of the soil and the pull 

 of the plant." If this be true, one plant is as good as another for 

 indicator use, and Kubanka wheat will furnish just as serviceable 

 results as would Adenostoma. This is fortunate, since difficulty has 

 been encountered in bringing about germination of the chaparral 

 species under artificial conditions. The one chief essential to 

 success seems to be the maintenance of uniform environmental 

 conditions during the period of the experiment, especially those 

 affecting the evaporation-rate. It must never be forgotten that 

 the wilting coefficient is the ultimate point at which plants must 

 wilt, but that under certain conditions wilting may take place 

 before that point is reached. Its usefulness in the investigation of 

 different soils with regard to their effect upon plant life is obvious. 



Comparison was made primarily between the soils of two stations, 

 Nos. 7 and 10, representative of forest and chaparral respectively. 

 In each case two depths were considered, 10 and 100 cm. Station 2 

 was added because of the interesting singularity apparent in its 

 other characteristics. The figures in table 8 are each an average 

 of seven unimpeachable determinations. 



The figures are about what one would expect after study of the 

 physical character of the soils and their water-retaining capacities. 



