8 THE RELATION OF DESERT PLANTS TO 



The caliche layer is perhaps an incrustation brought about by evap- 

 oration beneath the soil surface. In the dry season the soil becomes 

 air-dry to a considerable depth, and in this condition water must diffuse 

 as vapor through the interstices of the soil more rapidly than liquid water 

 can move from the moister layers below to the drier ones above. The 

 result is that the evaporating surface of the soil is often, and for long 

 periods, far below the soil surface, and, from this subterranean evap- 

 orating surface, water vapor diffuses upward through the dry soil- 

 layers to the air. As is well known, the soil of these regions contains a 

 large quantity of soluble salts. This soil solution, being lifted by evap- 

 oration, becomes concentrated, and finally the salts should crystallize 

 out at or near the evaporating surface. In this way the caliche hard- 

 pan may have been formed. Another hypothesis to explain the exist- 

 ence of this hard-pan supposes the caliche to have been formed at the 

 lower limit of penetration for precipitation water, the salts having 

 been gradually deposited as the soil was alternately wet and dry. To 

 definitely determine which of these hypotheses is more probable will 

 require further investigation. 



Just as the caliche underlies practically the whole surface of the 

 desert mesa, so too the crevices and fissures on Tumamoc Hill are 

 largely closed by a similar formation at the depth of a meter or less. 

 Plant roots penetrate into the cracks of this hard-pan both on the hill 

 and on the mesa, and it is probably a very important factor in conserv- 

 ing the meager water supply. 



WATER CONTENT OF THE SOIL IN THE DRY SEASON. 



At the time of the beginning of the work, July 1, 1905, the desert 

 conditions on the hill were nearing their maximum for the year. The 

 surface soil about the Laboratory building was air-dry and seemed 

 thoroughly baked. Day and night air temperatures varied from 80 to 

 105 F. or above, and the relative humidity of the day time varied 

 between 8 per cent and 15 per cent of saturation. Rain had not fallen 

 since May 12, at which time 1.97 cm. fell, and strong breezes or even 

 gales were almost constant. The only plants which remained in good 

 condition were those which are particularly adapted in some manner to 

 dry habitats. The giant cacti had just finished flowering and were 

 ripening their pulpy fruits, much sought after by Mexicans and Papagos 

 and even by groups of American children who now and then came out 

 from Tucson. Prickly pears were also ripening their fruits, while bar- 

 rel cacti and several arborescent opuntias showed no marked growth 

 and of course bore no leaves. The creosote bush was green but not 

 growing, and was covered with ripe fruits; many plants of ocotillo had 



