110 THE PLANT WORLD. 



RELATIVE TRAA^SPIEATION IN CACTI.* 



By Dr. Burtojs" Edward Livingston. 



It has been the method of physiologists to relate daily vari- 

 ations in the rate of water loss from plants to some physiological 

 activity of the organism, withont considering the seemingly ob- 

 vious fact that the evaporating power of the air varies throngh- 

 ont the day, being relatively large for the hours of light and rela- 

 tively small for those of darkness. Considering this last fact, 

 it would be expected that, if transpiration were merely a phys- 

 ical process, unmodified by organic activity, the curve of this 

 function would take a form very similar to the one which it is 

 known to exhibit. If a dish of water be allowed to stand in the 

 open and l)e weighed every two or three hours, the curve of its 

 rate of water loss will be found to confonn quite closely to the 

 published curves of transpiration rate. In order to determine 

 whether there is indeed any physiological regulation of the rate 

 of transpiration it is necessary to refer each curve of this func- 

 tion to the corresponding curve of physical evaporation, and to 

 determine how one changes its direction as related to the other. 

 In Publication Xo. 50 of the Carnegie Institution, attention has 

 been called to the fact that the ratio of the rate of transpiration 

 to that of evaporation, for the same place and for the same short 

 period of time, furnishes us with a true measure of these varia- 

 tions in the transpiration rate which are due to causes other than 

 changes in the evaporating power of the air. This ratio has 

 been termed the rate of relative transpiration, the term rate of 

 absolute transpiration being used to denote rate of transpiration 

 in the old sense, namelv the observed rate of water loss from the 

 plant. 



Curves or polygons constructed from the rates of relative 

 transpiration and of evaporation, for short periods throughout 

 the day, bring out the fact that for most plants the former rate 

 usually reaches its maximum somewhat earlier in the day than 



*The essentials of the present paper were presented before the 

 recent meeting of the Botanical Society of America, at New York. 



