TRANSPIRATION BEHAVIOR OF RAIN-FOREST PLANTS. 



99 



evaporation of the late afternoon and early night, and in the absence 

 of light — which is always to be reckoned with in its immediate effects 

 on transpiration — the conditions of stomatal openness are capable 

 of an apparent regulation of relative transpiration. 



Table 39. — Influence of darkness on transpiration. 



Amounts of transpiration and relative transpiration for Pilea nigrescena and Peperomia turfosa 

 in the diffuse light of the laboratory and in a dark chamber. Humidity was determined 

 by psychrometer and by polsoneter. 



INFLUENCE OF DARKNESS ON TRANSPIRATION. 



The securing of relative transpiration rates is of great value in the 

 investigation of the influence of individual factors on the rate of 

 transpiration. It is impossible, for example, to determine the rate of 

 transpiration of a plant in the light and then to place it in darkness 

 without changing other factors than the light. Such changes, notably 

 in air movement and humidity, are of strong influence upon the rate 

 of absolute transpiration, but without influence on the relative rate. 

 I was interested in the influence of darkness on transpiration in con- 

 nection with the general question of stomatal behavior and in connec- 

 tion with the relation between the diurnal and nocturnal transpiration 

 activities of rain-forest plants. With the means at hand to secure 

 relative transpiration rates, I made two tests of the rate for plants 

 placed first in the diffuse light of the physiological laboratory, and 

 afterwards in the dark chamber which has been described. 



