76 RELATION OF DESERT PLANTS TO SOIL MOISTURE AND EVAPORATION. 



(7) Immediately following germination, the seedlings of desert 

 plants exhibit a slow aerial growth, but an exceedingly rapid downward 

 elongation of the primary roots, so that these should soon attain to 

 depths where moisture is always present in adequate amount for growth. 



(8) The high moisture-retaining power possessed by the soil of 

 Tumamoc Hill holds near the surface much of the water received 

 from single showers and offers excellent opportunity for the rapid 

 absorption of this by such shallow rooting forms as the cacti. 



(9) The sap of Cereus, Echinocactus, and Opuntia exhibit osmotic 

 pressures no higher than those commonly found in plants of the humid 

 regions. 



(10) The effect of air currents in increasing evaporation and trans- 

 piration rates is so great that measurements of natural transpiration 

 can not be made in closed chambers. 



(11) By means of a new method involving a newly devised evapo- 

 rimeter, a physiological regulation of the rate of transpiration was 

 unquestionably shown to exist in the forms studied. The mechanism 

 of this regulation has not been studied. 



(12) The regulation of transpiration seems to be controlled by air 

 temperature, the checking of water loss beginning to be effective 

 between 79 and 90 F. , and the check being removed between 75 and 

 80 F. 



(13) The ratio of transpiration rate per unit leaf surface to evap- 

 oration rate per unit water surface is termed relative transpiration. 

 Relative transpiration is reduced by the regulatory response from unity 

 in the high periods to from one-twelfth to one-sixth in the low periods. 



