A Study of Evaporation and Plant Distridution. 215 



water surface has been that of a nearly l:emis]:)herical copper 

 pan 25 cm. in diameter and 12 cm. deep, filled to within 3mm. 

 of the edge at each daily reading. It has been found that the 

 standard porous cup loses one cul)ic centimeter of water for 

 each 0. 1416 mm. of de]:>th lost by this pan, indoors and under sum- 

 mer conditions at Tucson. 'This constant proved to be 137 for 

 the conditions of June, 1909, when the two-foot pan used bv 

 the Weather Bureau at .Salton, California, was taken as standard 

 for depth measurements. Inasmuch as different pans of water, 

 under different conditions exhibit different relations in their 

 rates of water loss, not only to the losses from the porous cup 

 but also to one another, there seems to be no advantage in con- 

 sidering evaporation measurements in terms of dejDth, and the 

 criterion in terms of cubic centimeters of loss per time period 



Figure 1 

 from the standard porous cup is as satisfactory as any other. 



This is especially true when we consider that the cup be- 

 haves, as far as the evaporation of water is concerned, more 

 like a plant than does a free water surface, and we are here pri- 

 marily interested in the evaporating power of the air as it 

 affects plant transpiration. 



To obtain a clear picture of the geographical variations 

 in the summer evaporating power of the air, as brought out in 

 the table, reference may be made to the four accompanying 

 profiles (Figs. 1-2) and to the evaporation chart of the United 

 States (Fig. 3.). The profiles aim to present graphically the 

 changes in summer eva]:)oralion intensitv which would be en- 

 countered in crossing the country from west to east and from 

 north to south. The graphs express average weekly loss in 



