PART I. 

 TRANSPIRATION AND STOMATAL MOVEMENT. 



The problem here considered has, as I conceive it, two aspects : 



(1) The degree of correlation between the normal rates of transpiration 

 and the normal changes in the size of the stomatal openings. There is, how- 

 ever, a logical difficulty to be encountered in that, even should a close corre- 

 lation be shown to occur, it would remain unproved that the movements 

 of the stomata were immediately related, in the causal sense, to the rise and 

 fall of transpiration rate. This must be fully reckoned with in attributing 

 a regulatory function to the stomata, for, unless marked quantitative differ- 

 ences between the functions of the chloroplasts of the chlorenchyma and 

 those of the stomata exist, it would be quite possible that the photosynthetic 

 activity of the chlorenchyma and of the guard-cells would run parallel without 

 a necessary causal relation between stomatal movements and changes in 

 transpiration rate. That such a causal relation exists is to be doubted from 

 the consideration that the stomata, in very many cases at least, have a 

 greater capacity for the outward diffusion of water-vapor than actually has 

 been found to occur (Brown & Escombe, 1900) and this may be true at any 

 given diffusion capacity of the stomata, depending upon their dimensions. 



(2) Since, in the final analysis, light, above all other factors, influences 

 stomatal movement, and since also appropriate illumination is the condition 

 par excellence under which photosynthesis takes place, if the supposed causal 

 relation under consideration exists, wide and sudden changes in the degree 

 of illumination should be accompanied by changes in transpiration rate, 

 and these, at the same time, by corresponding changes in the diffusion capac- 

 ity of the stomatal openings. 



If such changes of transpiration rate occur unaccompanied by stomatal 

 movement, the answer to the problem is clear. If, however, any changes 

 in the stomata occur, due consideration must be given, in the interpretation 

 of the data, to the law of diffusion through a perforated septum as formulated 

 by Brown & Escombe (loc. cit.}, whose study of the static diffusion of gases 

 through membranes perforated by minute openings led them to the surprising 

 result that the rate of diffusion through such openings is much greater than 

 would at first be expected. 



31 



