798 Lloyd, The Desert Botan. Laboratory of the Carnegie Instit. of Washington. 



servation of soil moisture is largely due to the high rate of evapo- 

 ration and the consequent formatioji of a dust niulch. It is partly 

 due to the presence of rock fragments and of a hard pan ealled 

 caliche. 



Desert forms show an adaptation to existence in dry soil, being 

 able to exist in soils somewhat dryer than those needed by plants 

 of the humid regions. This adaptation is however comparatively 

 slight, and cannot be considered of prime importance. 



The downward penetration of the water of precipitation is slow 

 through the adobe soil itself, but comparativlely rapid on the wholc, 

 011 account of the presence of numerous oblique rock surfaces along 

 which the flow is not markedly impeded. 



By the middle of the summer rainy season all of the soil ex- 

 cepting the first few centimeters is sufficiently moist to allow ger- 

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

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



The seeds of Fouquieria splendens and of Cereus giganteus fail 

 to show any special adaptation to germination in soils dryer than those 

 needeed by seeds of such mesophytes as TriUcum and Phaseolus. Im- 

 mediately following germination the seedlings of desert plants exhibit 

 a slow aerial growth but an excedingly rapid downward elongation 

 of the primany roots so that these should soon attain to depths 

 where moisture is always present in adequate amount for growth. 



The high moisture retaining power possessed by the soil of 

 the laboratory 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 rooted forms as the cacti. 



The saps of Cereus, Echinocactus and Opuntia exhibit osmotic 

 pressure no higher than those commonly found in plants of the 

 humid regions. 



The effect of air currents in increasing evaporation and tran- 

 spiration rates is so great that reliable measurements of transpiration 

 cannot be made in closed Chambers. 



By means of a new method, involving a newly devised eva- 

 porimeter, a physiological regulation of the rate of transpiration 

 was shown unquestionably to exist in the forms studied. This 

 regulation appears to be most probably due to stomatal action 

 responses. 



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. 



The ratio of transpiration rate per unit of leaf surface to eva- 

 poration rate per unit of water surface is termed relative transpi- 

 ration. Relative transpiration is reduced by the regulatory response 



