GENERAL SUMMARY. 1 37 



any interruption of the diminution of the stomatal dimensions. Such con- 

 clusions afforded sufficient evidence to throw real doubt on the accuracy of 

 the view of stomatal regulation as above indicated, and the problem was 

 then attacked from the vantage-ground of experiment. 



By this means it has been found that the rate of transpiration may undergo 

 sudden and wide changes without the accompaniment of a sufficient change 

 in the dimensions of the stomata to account for them on the theory of stomatal 

 regulation of transpiration. This generalization is in full accord with the 

 conclusions of Brown & Escombe in regard to the diffusion capacity of stomata, 

 which, in the particular plant studied by them {Helianthus), is 6 times that 

 of the actual maximum transpiration rate observed. A comparison of the 

 physical characters of the stomata of Fouquieria splendens and Helianthus 

 annitus shows that the diffusion capacities of the stomata of these two plants 

 are nearly the same. From this fact and from the observed maximum trans- 

 piration rate of Fouquieria, it appears that the possible diffusion capacity of 

 the stomata for water- vapor is probably never reached in this plant. And 

 from the observation that wide positive changes in the transpiration rate may 

 occur at any particular size of the stomatal pore, it must be further concluded 

 that the full diffusion capacity at such dimensions is in excess of the actual 

 diffusion. It appears, therefore, that stomatal regulation of transpiration 

 does not occur, though, of course, conservation of the contained water follows 

 on complete closure of the stomata. It is open to doubt, however, if this 

 condition ever obtains. As a corollary to the above it must be held that the 

 vapor pressure within the leaf varies from time to time, and that it does not 

 remain constantly at the point of saturation. 



INDUCED RHYTHM IN TRANSPIRATION. 



Observation of changes in the rate of transpiration during constant condi- 

 tions of temperatures and relative humidity, and in total darkness, brought 

 to light evidence that there exists in Fouquieria splendens an induced rhythm 

 in transpiration. The rhythm maximum falls at an early hour in the morning 

 at, or somewhat later than, the hour of normal stimulation by the rising sun. 

 The evidence is lacking that this maximum is related to stomatal movement, 

 as, on the grounds stated above, the amount of change in the rate of transpi- 

 ration involved in the attainment of the rhythm maximum is too small to be 

 attributed to change in the dimensions of the stomatal pores ; and inductive 

 evidence has been brought to the support of this. The conclusion follows 

 that the observed induced rhythm is a true phenomenon of transpiration, the 

 cause for which is quite obscure. It may be remarked that the relegation of 

 this behavior to the category of rhythm is the best disposition that we may 

 make at this time. 



