548 Mr. Horace T. Brown [March 22, 



the question of how far these minute openings play a part in the 

 interchanges of gases between the interior of the leaf and the outer 

 air, has been a subject of very lively controversy. 



It is now about thirty years since the eminent French chemist, 

 Boussingault, came to the conclusion that the carbonic acid of the air 

 gains access to the leaf, not through the stomates, but through the 

 continuous substance of the cuticle and epidermis, by a process of 

 osmosis similar to that by which carbonic acid had been shown by 

 Graham to pass through a thin film of india-rubber. 



So convincing did Boussingault's experiments and arguments 

 appear to his contemporaries that this view became an article of 

 faith for something like a quarter of a century, until in fact, some 

 five or six years ago, when Mr. F. Frost Blackman took up the subject 

 and proceeded most inconsiderately to shatter all the most cherished 

 statements of our text-books on this question. 



I regret that time will not allow me to do more than state the 

 general conclusions at which Mr. Blackman arrived, and which may 

 be briefly summarised as follows : — 



In the first place there is no appreciable passage of atmospheric 

 carbonic acid through the surface of a leaf which is naturally devoid 

 of stomates, such for instance as the upper surface of a normal leaf, 

 which is quite imperforate ; neither is any entry of carbonic acid 

 possible when the stomates have been artificially blocked, or made to 

 close spontaneously. 



In addition to this, if a leaf has stomates on both surfaces, the 

 intake of carbonic acid by those surfaces bears a distinct relation 

 to the distribution of the stomates. 



We can, in fact, no longer doubt that when a leaf is respiring or 

 assimilating, mere osmosis of carbonic acid through the substance of 

 the cuticle and epidermis plays little or no part in the gaseous ex- 

 changes, and that, whatever the exact nature of the process may be, it 

 must be carried on exclusively by the minute openings of the stomates. 



Since anything like a mass movement of the air through these 

 openings is out of the question, we must look to the phenomena of 

 diffusion for the true explanation, and especially to that form of it 

 which was first described by Graham as free diffusion, that is to say, 

 the natural tendency possessed by gases or liquids to form a perfect 

 mixture when they are in contact with each other and there is no 

 partition of any kind between them. 



This spontaneous mixing is quite independent of any currents or 

 mass movements of any kind, and is brought about by the gradual 

 interpenetration of the molecules of the one gas or liquid by the 

 molecules of the other. 



As an example of this kind of diffusion, I have here a cylinder 

 which, a few weeks ago, was partly filled with a 5 per cent, gelatine 

 solution. After the gelatine had set, the cylinder was filled up with 

 a highly coloured solution of copper salt, which you now see has 

 permeated the jelly to a certain depth. There has been no mixing 



