110 BURTON EDWARD LIVINGSTON 



rounding the instrument to remove water vapor from a specified 

 surface exposed in a specified way. Evaporation data cannot be 

 interpreted, then, excepting in terms of the particular form, size, 

 etc., of instrument that is used. 



This feature has given rise to considerable misunderstanding 

 in regard to the various instruments and the value of their read- 

 ings. It has frequently been thought, for example, that several 

 porous cup atmometers operated at different places show the 

 comparative rates of transpiration of plants growing in the 

 vicinity of the respective instruments. While this conclusion 

 may sometimes be approximately true, it does not necessarily fol- 

 low at all. In order that this may be true it would be necessary 

 that the plants expose surfaces for evaporation having always 

 the same properties as the surfaces of the instruments, and that 

 instruments and plants be similarly exposed As a matter of 

 fact, we know that plant surfaces are continually altering in their 

 transpiring power 26 and that they are not the same from hour to 

 hour and from day to day. Since the atmometer surface remains 

 constantly of the same water-supplying power (if this is not 

 approximately true or cannot be assumed after the corrections 

 have been made, it is useless as an atmometer) , it is clear enough 

 that actual plant transpiration cannot be calculated from at- 

 mometric data alone. 



On the other hand, the exposure, surface, etc., of the porous 

 cup atmometer are so nearly similar to the corresponding fea- 

 tures of plants in general, that the instrument offers the only 

 means so far available for studying the evaporating power of the 

 air as it generally affects the rate of plant transpiration. Atmom- 

 eters are employed in this connection to measure an environmental 

 condition and their readings are indices of the power of the air 



26 Livingston, B. E., and Brown, W. H., Relation of the daily march of trans- 

 piration to variations in the water content of foliage leaves. Bot. Gaz. 53: 309- 

 330, 1912. 



Livingston, loc. cit., 1900. 



The resistance offered by leaves to transpirational water loss. Plant 



World 16: 1-35, 1913. 



Bakke, A. L., Studies on the transpiring power of plants as indicated by 

 the method of standardized hygrometric paper, .lour. Ecol. 2: 145-173, 1913. 



