128 REPORTS ON INVESTIGATIONS AND PROJECTS. 



Livingston, Burton E., Bureau of Soils, U. S. Department of Agri- 

 culture. Grant No. 156. Investigation of the relations of desert 

 plants to soil moisture and to evaporation. (For first report see 

 Year Book No. 3, p. 100.) $400. 



Dr. L/ivingston reports that the work is completed and gives the 



following summary of results : 



(1) The deeper soil layers of the laboratory hill contain at the 

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

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

 throughout the months of drought. 



(2) This conservation of moisture in the soil is largely clue 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 a 

 hard-pan formation 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 adobe soil itself, but comparatively rapid on the whole, 

 on account 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 

 germination and growth of most plants. The surface itself is often 

 wet for periods of several days during the season of summer rains. 



(6) Seeds of Fouquieria 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. 



(7) Immediately following germination the seedlings of desert 

 plants exhibit a slow aerial growth, but an exceedingly rapid down- 

 ward elongation of the primary roots, so that these 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 the 

 laboratory hill holds near the surface much of the water received from 

 a single shower and offers excellent opportunity for the rapid absorp- 

 tion of this by such shallow- rooting forms as the cacti. 



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

 pressures no higher than those commonly found in plants of the 

 humid regions. 



