Physiologie. 185 



Livingston, B. E. f The relation of descrt plants to soil 

 moisture and to e vapo ratio n. (Carn. Instit. Washington, 

 Public. L. p. 1—78. Fig. 1 — 16. Aug. 1906.) 



Comprising soil, atmosphcre and plant studies in the vicinity 



of Tuscon, Arizona. A sammary of the principle results as 



given by the author 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 content 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 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 comparativety 

 slight and cannot 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 

 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 sufflciently moist to allow T 

 germination 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 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 Triticwn and 

 PJiaseolus. 



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 should soon 

 attain to depths where moisture is always present in adequate 

 amount for growth. 



S. 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 Opuniia 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 

 transpiration rates is so great that measurements of natural transpi- 

 ration can not be made in closed Chambers. 



11. By means of a new method involving a newty devised 

 evaporimeter, a physiological regulation of the rate of transpiration 

 was unquestionably shown to exist in the forms studied. The mecha- 

 nism of this regulation has not been studied. 



12. The regulation of transpiration seems to be controlled b)r 

 air temperature, the checking of water loss beginning to be effec- 

 tive 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 eva- 

 poration rate per unit water surface is termed relative transpiration. 



Relative transpiration is reduced by the regulatory response from 



