24 THE RELATION OF DESERT PLANTS TO 



not be likely to escape into the air to any great extent, excepting 

 through the transpiration of plants. A discussion of the conservation 

 of moisture by this soil will be given under the succeeding heading. 



ATMOSPHERE STUDIES. 

 GENERAL PROBLEM. 



Aside from the ravages of animals, desert conditions in the locality 

 under discussion are brought about mainly by two different, though 

 related, factors dry ness of the soil and excessive evaporating power of 

 the air. The former factor offers resistance to the absorption of water 

 by plant roots, and the latter accelerates water loss by transpiration 

 from the leaves and stems, so that both factors work together to bring 

 about the extreme xerophytism so manifest everywhere in the aspect 

 of the vegetation. 



The soil conditions have been discussed in the previous section. 

 There will now be presented the results of some measurements of the 

 evaporating power of the air. This depends upon two conditions rela- 

 tive humidity and air movements. Temperature variations affect the 

 evaporation rate through changes in relative humidity. Relative 

 humidity acts directly through alterations in the vapor tension of water. 

 As is well known, a wind increases the rate of evaporation very mark- 

 edly by furnishing a constantly renewed air layer against the evapo- 

 rating surface and thus preventing, to a greater or less degree, the local 

 rise in relative humidity which would otherwise occur. Both of these 

 factors are of the utmost importance in influencing the transpiration of 

 plants, and the latter deserves more attention than has heretofore been 

 given to it by most plant physiologists. The only important investi- 

 gations of the effects of air currents upon the transpiration rate are 

 those of Wiesner (1887), who has shown that, while in certain cases 

 wind causes a closing of the stomata, it usually does not have this effect, 

 but causes a marked rise in the rate of water loss. Eberdt (1889) has 

 corroborated these results of Wiesner. 



The data obtained in regard to the evaporating power of the air will 

 be given under two headings: 'Evaporation from a water surface," 

 and ' ' Evaporation from the soil. ' ' 



EVAPORATION FROM A WATER SURFACE A NEW FORM OF 



EVAPORIMETER. 



Measurements of evaporation are usually made by direct determina- 

 tion of water loss (in terms of either volume or weight) from some sort 

 of vessel of water, the upper surface of which is open to the air. Even 

 though comparatively small readings can be taken, this method is not 



