DEPARTMENT OF BOTANICAL RESEARCH. 73 



at an3'- point, to remove water by evaporation from a liquid surface 

 may be studied quantitatively by means of any form of atmometer 

 that is able to supply new liquid wate/ to the exposed surface as rapidly 

 as drying occurs through evaporation. But the measurements must 

 always be considered as related to the particular form of atmometer 

 used. It is not possible, for example, to calculate evaporation from an 

 open pan of water by means of readings secured with a porous-cup 

 atmometer, etc. This feature makes the study of evaporation as a 

 climatic feature very complex and somewhat more difficult than is the 

 corresponding study of rainfall. To gain more informaMon on this 

 subject, pans of water were employed with several forms of porous-cup 

 and other atmometers in a series of observations made at Tucson in the 

 summer of 1921. The results support the conclusions previously 

 reached, and the white-porcelain sphere remains the most satisfactorj'' 

 form of atmometer for the careful study of the evaporating power of 

 the air, at least aside from the influence of sunshine. For less detailed 

 and less precise studies, open pans of liquid water and non-spherical 

 porous cups have their places, and the Bates cloth-wick atmometer 

 (which was included in the tests here considered) appears to be satis- 

 factory for roughly general comparisons when the evaporating power 

 of the air does not exceed the capacity of the instrument to deliver 

 liquid to the evaporating surfaces as these are exposed through the 

 perforations in the lower plate. The last-named instrument appears 

 to be suitable, as far as its readings are concerned, for the ecological 

 and forestry work for which it was devised. 



Plant transpiration is evaporation from water-imbibed cell-walls, 

 and the imbibed water contains significant amounts of dissolved 

 material. It has often been suggested that the rate of plant trans- 

 piration may be considerably retarded by high solute concentration in 

 the cell solution, this suggestion being deduced from the fact that 

 such high concentration of a solution is accompanied by high osmotic 

 potential and by lowered vapor tension. It has been pointed out, 

 however, that such concentrations as generally occur in plant tissues, 

 while they may sometimes give osmotic values as high as 20 or 30 

 atmospheres, can produce only a slight lowering of the vapor pressure 

 of the solutions as compared with water. Although this matter seems 

 quite clear from the physical view-point, some tests with pans of water 

 and of molecular cane-sugar solution were made at Tucson in 1921, 

 to determine the relative evaporation rates of these two Uquids when 

 similarly exposed. From these tests it appears that, in the absence of 

 direct sunshine, the sugar solution evaporated at a rate less than 10 per 

 cent lower than the rate at which water evaporated under the same 

 conditions. This is in accord with physical theory. But another 

 point was brought out, namely, that when both the pans were exposed 

 to direct sunshine the sugar solution evaporated considerably more 



