1 8 ROOT HABITS OF DESERT PLANTS. 



1 1 the sample at a depth of lo cm. had 17.3 per cent water, and that from 

 a depth of 20 cm. had 18.6 per cent its dry weight. Ten days later the 

 percentages had fallen to 11.7 and 14.3, respectively, for the two depths. 

 Following this date the amount of water diminished rapidly until, four 

 weeks after the maximum water content, it was no more than immediately 

 before. The readings on March 12 for the depths of 10 and 20 cm. w^ere 

 7.9 and 9.5 per cent dry weight. This was considerably under the amounts 

 of water found on Tumamoc at the time and shows the bajada to be the 

 more arid of the two areas. 



The period during which the adobe clay of the bajada at this situation 

 contained sufficient water to be of use to shallow- rooted plants was thus 

 not more than three weeks of the year in question. This period would 

 probably be somewhat extended and might be comparable to the Tumamoc 

 Hill soils in portions of the bajada where the soil contains less rock and 

 caliche fragments and also has the advantage of position near higher areas. 

 The brevity of the period in which the bajada soil contains sufficient water 

 for the growth of annuals, or of perennials without water-storage organs, 

 is therefore an important feature of the xerophilous conditions of this 

 physiographic area and indicates that it is the most intensely arid of any 

 examined. 



THE WEST WASH. 



Tumamoc Hill, as above noted, is situated apart from the main range of 

 the Tucson mountains. At both the east and the west bases of the Hill 

 are washes, of which the former heads in the hill, while the latter receives 

 drainage also from portions of the main range. The West Wash, therefore, 

 drains a large area and during seasons of heaviest storms carries a large 

 amount of water which may overflow its banks and flood the bordering 

 plain. 



The soil of the wash is coarse sand and allows water to percolate through 

 it very rapidly. It is possible that there is a water table underlying the 

 wash and its flood-plain, but this has not been demonstrated. The sandy 

 loam of the plain has a water capacity of about 25 per cent its dry weight, 

 or considerably higher than that of the upper reaches of the bajada. 



The soils of the plain only have been studied; the studies were made 

 from October, 1907, to April, 1908. The studies on the loams of the plain 

 showed that up to February 11 they contained less than 10 per cent of 

 water. On February 11 the soil at a depth of 15 cm. contained 14.9 per 

 cent. At a depth of 30 cm. the soil during the entire period did not con- 

 tain over 8.2 per cent water. On March 2 the soil at the lesser depth had 

 9.3 per cent and that at the deeper location 6.2 per cent water, April 11 

 the percentages of water for the two levels were, respectively, 5.3 and 4.6. 

 The dryness of the upper soil is due to its sand-loam nature, which permits 

 the rapid sinking of water and its rapid evaporation. In this place the 

 soil is over 2 meters deep, and had studies been carried out on the deep 



