30 



THE RELATION OF DESERT PLANTS TO 



itself, the stationary wet and dry bulb thermometer, placed 

 in an open position where air currents may affect it, is the 

 most reliable instrument for determining evaporating power. 

 Relative humidity computed from readings of this instrument 

 should not be the same as when com- 

 puted from psychrometer readings, 

 but should bear a closer relation to 

 the losses from the evaporimeter. 



The power of air currents to ac- 

 celerate evaporation was constantly 

 observed in the progress of the work 

 at the Desert Laboratory. This pow- 

 er is noticeable in regard to transpi- 

 ration from the plant surfaces, as 

 was observed repeatedly in exper- 

 iments where weighings of potted 

 desert plants were made at short 

 intervals, the plants stand- 

 ing in the open on a shelf 

 near the cylinder of the evap- 

 orimeter so that transpira- 

 tion rates could be compared 

 with those of water loss from 

 the instrument. When the *' 

 air was quiet the rates of 

 both transpiration and phys- 

 ical evaporation were com- 

 paratively low, while the 

 rates rose immediately when 

 even a breeze sprang up and 

 always reached their max- 

 ima for any given tempera- 

 ture during the heavy gales 

 which often blew over the 

 hill for hours. It therefore 

 appeared that the 

 transpiration figures 

 obtained by Spalding 

 (1904) by means of 

 the bell- jar method 

 are uniformly far too 

 low to represent nat- 



M 



81 ^ 



06 <6 



66 Q. 



K 



0> 



K 



un 



00 



) 



FIG. 5. Curves of temperature and rate of evaporation, July 24-26, 1904. 



