60 VEGETATION OF A DESERT MOUNTAIN RANGE. 



of their ranges. It was impossible, nevertheless, to secure a set of 

 soil samples in the humid mid-summer which would be representative 

 of the maximum moisture conditions and at the same time comparable 

 for the various altitudes. A set of samples taken at the same interval 

 after a rain of the same amount, at each of the several elevations, 

 would comply with the requirements. 



All samples of soil for moisture content were taken from a depth 

 of 15 cm. The conditions at this depth are of importance for ephemeral 

 herbaceous plants and for some shrubs, but the trees and larger shrubs 

 are, of course, dependent for their supplies on much more deep-seated 

 bodies of soil. The rocky character of the substratum means that the 

 largest perennial plants are dependent to a great extent upon the 

 moisture contained in the soil which occupies the crevices of the rock 

 in situ. It is particularly noticeable that the lowest trees of the Encinal 

 region grow in the uppermost part of talus slopes or along the bottoms 

 of cliffs. In such situations it is doubtless possible for the roots of 

 these trees to reach soil-filled crevices which are fed by gravity with 

 the water of large veins of soil above. 



The samples of soil were secured by digging with a hand trowel and 

 transferring quickly to bottles, which were tightly stopped, and then 

 coated over the stopper with vaseline. The soils were dried in the 

 original bottles by heating to 100 C. until they showed constant 

 weight. The percentages of moisture have been calculated on the dry 

 weight as unity. The physical texture of all samples taken was very 

 similar, but there was a greater amount of humus in those from the 

 higher elevations. 



Three series of soil samples were taken at various times to determine 

 the conditions prevailing in the arid fore-summer. These samples 

 were taken at 1,000-foot intervals, from the vicinity of the rainfall 

 stations, and were secured in pairs, one sample being from a south 

 slope and one from a north slope. The localities chosen for sampling 

 were typical of the slopes at the several elevations, and in every case 

 the pair of samples was secured in the midst of the dissimilar vegeta- 

 tions which occupy the opposed slopes. 



On April 27 to 29, 1911, a series was secured from 3,000 feet to 7,000 

 feet (see table 7) . For the three months preceding the taking of these 

 samples there had been only light and infrequent rains over the sur- 

 rounding region, the rainfall of the mountains themselves for this period 

 being unknown. At Tucson there was a rainfall of 0.28 inch on April 

 2, and there was no appearance of rain on the mountains after that 

 date. On June 9 to 11 another series of samples was secured at the 

 same stations, together with a pair from the station at 8,000 feet. 

 There had been no rain between the securing of the two sets of samples. 



A comparison of the percentages of moisture in April and in June 

 shows them to be of about the same order of magnitude. The relative 



