14 ROOT HABITS OF DESERT PLANTS. 



formation. No physiographic area of this vicinity has so great diversity 

 as the bajada. 



The portions of the bajada brought into this study comprise the upper 

 slope at the north base of Tumamoc Hill, the drainage slopes west of West 

 Wash, the high mesa about a mile east of the vSanta Cruz river, a certain 

 locahty about 15 miles east of Tucson, and the upper reaches of the bajada 

 at the west base of the Rincon mountains. The range of the bajada in 

 altitude at the stations mentioned runs from 2,500 feet, at the north base 

 of Tumamoc, to about 3,500 feet at the west base of the Rincons. 



The soils of the bajada are unhke in the localities mentioned, but a study 

 of them, aside from observations in the field, has been confined to that 

 portion which lies just north of Tumamoc Hill. The soils of the different 

 localities, however, have certain features in common, some of which are as 

 follows : The upper soil layer, to a depth of 30 cm., more or less, is of adobe 

 clay. Underlying the adobe is caliche, a calcareous hardpan, which extends 

 to an indefinite depth. The lower portion of the adobe, perhaps 10 cm., 

 is composed of fragments of caliche and frequently of rock, and for con- 

 venience is here referred to as "rotten" cahche. It is a common occur- 

 rence that the caliche hardpan is cracked, or is wanting in small areas, so 

 that the adobe which replaces it is consequently of considerable thickness. 

 Plate I, taken from a photograph of a cut in the bajada a mile east of the 

 river flood-plain, shows variation in the depth of the soil which had been 

 brought about in the manner indicated. The adobe is also of greater depth 

 where it has accumulated in depressions as a result of the erosion of higher 

 areas. 



Although certain general conditions arc shared by all of the bajada soils 

 examined, even a superficial examination and comparison of them shows 

 great diflferences. On the portions of the bajada where the distance from 

 the mountains is relatively great, the adobe is practically homogeneous; 

 but near the base of the mountains, as at the north base of Tumamoc, it 

 may be shot through with fine rock fragments and caliche which may about 

 equal half the volume of the whole. On the slopes west of West Wash 

 there is little adobe, and what is comparable to the rotten caliche of the 

 other parts of this formation there comes very close to the surface, thus 

 causing a larger percentage of caliche and rock fragments at this place. 



A special characterization of the soil conditions in the habitats of each 

 plant examined will be given with the account of the root-system of the 

 plant. 



