138 



PLANT SOCIOLOGY 



i 6 7 8 3 Wit n J3 mS 16}7I8JS 

 Fig. 75. — Diurnal range 

 of transpiration from Encilia 

 farinosa (T) and evapo- 

 ration from an atmometer 

 (£) on May 25, dry season, 

 at Tucson, Arizona. {After 

 Shreve.) 



(Fig. 74). It is impossible to deduce the water loss from plants 

 directly from evaporation, although transpiration and evaporation 

 curves will frequently be found to parallel one another rather closely 

 (Fig. 75). However, different plants respond quite differently to the 

 factors controlling evaporation because of the difference in the density 

 of cell sap and colloidal content of the living organisms. Transpira- 

 tion, however, stands in such an intimate relation to evaporation that 

 measurements of evaporation, along with precipitation, throw much 

 light on the water economy of plant communities.^ 



Measurement of Evaporation. — Meteor- 

 ologists usually have preferred to measure 

 evaporation from a free water surface of 

 a deep pan, giving the results in the depth 

 in inches or centimeters of water lost within 

 a given time. Evaporation is thus regarded 

 as the inverse of precipitation. Since the 

 water surface reflects the greater part of the 

 incident radiant energy, while colored bodies 

 like plants absorb it, the rate of water loss 

 from the water surface of the pan and 

 from plants is very differently affected by insolation. Then as the 

 water in the evaporating pan is deep, there is a very decided lag in its 

 temperature changes as compared with those of the atmosphere, while 

 the leaves of plants quickly adjust themselves to the same temperature 

 as the air. These difficulties have been largely overcome, however, 

 by the use of a shallow blackened pan from which Briggs and Shantz 

 (1917) found that evaporation closely paralleled the rate of tran- 

 spiration from Medicago sativa. 



Ecologists during the last two decades have found that measuring 

 instruments known as "atmometers" react to external factors in a 

 manner somewhat similar to the plant body and that atmometers 

 possess many advantages over the open-pan evaporimeter ; that is, 

 the atmometers measure rather accurately that summation of atmos- 

 pheric factors that tend to make demands upon the water within the 

 plant body. They can take no account, however, of the modification 

 of transpiration by stomatal control or by osmotic concentration of 

 the cell contents. 



The evaporating surface of the atmometer is a porous clay cup, 

 preferably of spherical form, fed from beneath from a water reservoir. 



^ A good account of the physics of evaporation and of the relationship of tran- 

 spiration to evaporation may be found in Maximov, "The Plant in Relation to 

 Water," pp. 133-155, 1929. 



