■ SOIL MOISTURE. 35 



Kther meteorological station in the United States, and the significance 

 of this low degree of humidity lies in the enormous capability for evapo- 

 ration possessed by air in such a condition. There are few plants which, 

 in a soil practically air-dried, have developed a sufficient amount of 

 absorptive root surface and a sufficiently small and slowly evaporative 

 leaf and stem surface to keep from drying up. 



CONSERVATION OF MOISTURE. 



In the examination of the characteristics of desert vegetation, one ot 

 the causative conditions most important to be kept in mind is the im- 

 mediate source of water supply for the plant. The winter and spring 

 rainfall, which is the primary source of moisture, has already been dis- 

 cussed, but the method of its conservation is of almost equal impor- 

 tance. 



The moisture contained in soils is commonly classified, according to 

 the mechanical method of its retention, as hydrostatic, capillary, and 

 hygroscopic. Hydrostatic moisture exists in the form of free water — 

 such, for example, as that, which rises in a well, or that which stands 

 beneath the surface of a swamp. Capillary moisture is that which the 

 soil has absorbed from any source by capillary attraction. The max- 

 imum of capillary moisture for any soil is the amount that a suspended 

 body of that soil will retain without dripping, while a zero amount is 

 contained in soil dried in atmospheric air. Hygroscopic moisture is 

 that which remains attached to the particles of air-dried soil. It is not 

 visible, but its presence may be demonstrated by heating the soil in a 

 kiln, when the moisture is given off as vapor. Conversely, hygroscopic 

 moisture is that absorbed by kiln-dried soil upon exposure to atmos- 

 pheric air. 



In the deserts of the Death Valley region permanent hydrostatic 

 moisture is rare. It exists about springs and the streams which flow 

 from them, and in the bottoms of some of the larger valleys. Where 

 such water is not intensely alkaline, or is situated only a few yards 

 beneath the surface, its presence is indicated by a growth of mesquite, 

 and if its depth is only a few feet other indicative plants occur. One 

 of them, Sporobolm airoides, locally known as bunch grass (a name ap- 

 plied unfortunately to many other plants as well), has come to be of 

 special importance to the desert traveler, for it indicates tbe presence, 

 within a i'e>\\ feet of the surface, of water sufficiently fresh to drink. 

 Such sources of water are, however, not to be considered in a discus- 

 sion of the desert proper. 



Capillary moisture also exists in the vicinity of springs and per- 

 manent underground water supplies; but in such cases, like the hydro- 

 static moisture from which it arises, it has no connection with the true 

 desert vegetation. 



Local rainfall alone, then, remains to be considered. As the rain is 

 precipitated it is taken up eagerly by the parched soil as capillary 



