SOIL MOISTURE AND TO EVAPORATION. 17 



shower. These plants begin to absorb water and to swell almost im- 

 mediately after the surface soil about their bases is wet either by rain 

 or artificially. The more sandy and gravelly soils of the surface of the 

 mesa at the foot of the hill possess this property of holding water to a 

 much less degree, and water falling upon them readily finds its way to 

 the lower levels and finally to the drainage channels of the Santa Cruz 

 River and its branches. No doubt this sandy character of the mesa 

 soil furnishes the main reason why the vegetation on the mesa here is 

 so much more xerophylous in character than that on Tumamoc Hill. 

 Between the base of the latter and the Santa Cruz sand-wash prac- 

 tically the only plant to be seen in the dry season is the creosote bush, 

 and the specimens of this shrub here found are not by any means so 

 vigorous as those growing in the clay soil of the hill. The same con- 

 dition of things is to be observed in the relation of the mesa vegetation 

 on the other side of Tucson to the vegetation which occupies the foot- 

 hills of the Santa Catalina Range. 



POWER OF THE SOIL TO RAISE WATER FROM LOWER LEVELS. 



The power of a soil to raise water, by capillarity, from the lower- 

 lying layers follows very closely its retaining power. The rate at which 

 this water movement takes place and the height reached by the water 

 above the source of supply depends primarily upon the smallness of 

 the capillary spaces of the soil, and hence upon the fineness of the 

 component particles as well as upon their degree of compactness. 



Capillary lifting power is most often measured by filling a vertical 

 glass tube with soil, placing its lower end in water, and measuring the 

 rate at which the water ascends the soil column, this being determined 

 by the change in the color of the soil as it becomes moist. A better, 

 though much slower, method for determining the maximum height to 

 which water will thus rise is to saturate a tube of the soil, place its 

 lower end in water, and determine the maximum depth from the upper 

 surface to which the soil becomes dry. The latter method was 

 attempted with the soil under consideration, but the author's time was 

 too limited to obtain any evidence therefrom. 



By the other method results were obtained which warrant presenta- 

 tion here, although the soil columns used were undoubtedly much less 

 thoroughly packed than is the natural soil. The afternoon of August 2, a 

 vertical glass tube of 1.8 cm. internal diameter was filled with air-dry 

 soil, after having its lower end closed by tying a layer of cloth over it. 

 The tube was tapped rapidly on the floor while the soil was slowly poured 

 in, so that the latter was fairly compact when the tube became 

 filled. The lower end of the filled tube was placed in a vessel of water 



