SOIL MOISTURE AND TO EVAPORATION. 75 



The experimental evidence is very consistently in favor of the idea 

 that air temperature is the controlling factor for the regulatory response 

 in question. It appears that with the rising temperature of the morn- 

 ing hours a physiological maximum is reached at which the rate of 

 relative transpiration begins to be checked, and that this response is 

 reversed and relative transpiration begins again to increase when the 

 air temperature has passed its daily maximum and has decreased to 

 another point which seems to be a physiological minimum. The latter 

 temperature appears to be somewhat lower than the physiological 

 maximum at which the check is imposed. This maximum occurs 

 between 79 and 90 F. , while the corresponding minimum occurs between 

 75 and 80 F. 



The regulative response produces a reduction in relative transpira- 

 tion from unity in the high periods to from one-twelfth to one-sixth in 

 the low periods. 



SUMMARY. 



The main results of these experimental studies may be briefly stated 

 as follows: 



(1) The deeper soil layers of Tumamoc Hill contain, at the end 

 of the spring dry season, and thus probably at all times, a water con- 

 tent adequate to the needs of those desert plants which are active 

 throughout the months of drought. 



(2) This conservation of soil moisture is largely due to the high 

 rate of evaporation and the consequent formation of a dust mulch. It 

 is partly due to the presence of rock fragments and of the hard-pan for- 

 mation called caliche. 



(3) Desert forms show an adaptation to existence in dry soil, 

 being able to exist in soils somewhat drier than those needed by plants 

 of the humid regions, but this adaptation is comparatively slight and 

 can not be considered of prime importance. 



(4) The downward penetration of precipitation water is slow 

 through the soil itself, but comparatively rapid on the whole, on ac- 

 count of the presence of numerous oblique rock surfaces along which 

 the flow is not markedly impeded. 



(5) By the middle of the summer rainy season all of the soil 

 excepting the first few centimeters is sufficiently moist to allow germi- 

 nation and growth of most plants. The surface itself is often wet for 

 several days at a time during the period of summer rains. 



(6) Seeds of Fvuquieria splendens and of Cereus giganteus fail to 

 show any special adaptation to germination in soils drier than those 

 needed by the seeds of such mesophytes as Triticum and Phaseolus. 



