NORMAL DAILY PERIODICITY. 



43 



ston (loc. cit.), constructed a curve of "relative transpiration," the character 

 of which, as compared with the "absolute transpiration" curve, led him to the 

 belief that "some physiological change in the plant {Euphorbia) effective 

 during certain hours" brings about a regulation of evaporation from the leaf 

 which results in a course of events different from that which takes place in 

 physical evaporation. Among other suggestions, the "response of the sto- 

 matal mechanism" is the one which concerns us in particular. The case of 

 Euphorbia will be referred to further on. With respect to Verbena, the issue 

 may be approached by means of the data given in the table 22. In it are 

 to be found the transpiration rates (absolute transpiration), the relative 

 transpiration rates, and the rates of evaporation, all collated by Dr. Living- 

 ston in July of the present year. The appropriate curves for relative trans- 

 piration and rate of evaporation are included in fig. 12. 



Table 22. — Transpiration and evaporation rates, Verbena ciliata. 



It is to be noted that the maximum absolute transpiration rate falls at the 

 same hour as that of evaporation. It is to be inferred from this that the 

 conditions which produce a high rate of evaporation cause the proportionally 

 rapid removal of water-vapor from the surface of the plant. It is significant, 

 too, that the rate of absolute transpiration is checked after the noon hour, and 

 that at this time is to be found the maximum of relative transpiration ; or the 

 check is to be noted in the decreased rate after this time. The rapid fall of 

 absolute transpiration after 3 p.m. is clearly due to the rapid decrease in the 

 rate of evaporation, but there is evident, also, a retarding influence at work on 

 the plant which brings the rates of absolute and relative transpiration closer 

 together than those of evaporation and absolute transpiration. To what 

 extent may this regulation be referred to stoma tal activity, however? 



The closed condition of the stomata, assuming complete closure, which, as 

 Francis Darwin believed, scarcely obtains, inhibits intercellular transpiration. 

 The total escape of water-vapor from the leaf must then occur through the 

 cuticle. Conversely, the fully open condition of the stomata permits a diffu- 

 sion which, under optimum conditions for internal physiological processes, 



