1901.] on Some Becent Work on Diffusion. 551 



renewed film of a strong solution of caustic alkali, submitted to a 

 strong current of air. 



This is, in itself, a somewhat remarkable conclusion, but what 

 are we to say to a proposition which would limit the absorptive 

 power of the leaf surface to the extremely small apertures of the 

 stomates ? 



In a leaf such as we have been considering, the aggregate area of 

 the openings of the stomates, when expanded to their widest, amounts 

 to less than one per cent, of the total leaf surface, so that if the entry 

 of the CO., takes place exclusively by these openings, we must con- 

 clude that it goes in more than fifty times faster than it would do if 

 the mouth of each one of these miuute openings were filled with a 

 constantly renewed solution of strong caustic alkali. 



Such facts make it difficult unreservedly to accept the view that 

 the gaseous exchanges in leaves are really carried on exclusively by 

 the stomates, which occupy such a small fraction of the leaf surface. 

 On the other hand, the direct experimental evidence in favour of this 

 view is overwhelming, so that we apparently find ourselves on the 

 horns of a dilemma. 



There appeared to be only one way out of the difficulty — that was 

 to assume that the leaf knows more about the laws of free diffusion 

 than we do, and bas adapted itself to some physical principles which 

 have hitherto escaped notice. This was found to be the case when 

 the structure of the leaf was regarded as a piece of physical apparatus 

 for promoting rapid diffusion. 



I do not propose to take you through the various and tedious 

 stages by which the true explanation was reached, but will attempt, 

 as far as possible, to short-circuit the current of the argument. 



In the first place, I wish to call your attention to a particular 

 mode of free diffusion which, in gases, has been but little studied, but 

 which has a very direct bearing upon diffusion in the living leaf, 

 where one of the constituents of the diffusing gases, the carbonic acid, 

 is very small in amount compared with the others. 



Let us for a moment concentrate our attention on the air which is 

 contained in this open glass cylinder, and endeavour to picture to our 

 minds the jostling crowds of the perfectly elastic molecules of the 

 various gases, flying hither and thither in all imaginable directions, 

 and coming into frequent collision with each other and the sides of 

 the containing vessel. 



Now in this jostling throng there is a certain proportion of mole- 

 cules of carbonic acid, which we will imagine for the moment are 

 distinguished from the molecules of the other gases by some differ- 

 ence in colour — let us suppose them to be green. 



Now, further, consider a plane surface in the contained air of the 

 cylinder ; from the dynamical theory of gases it follows that in any 

 given interval of time, temperature and pressure remaining constant, 

 the same average number of the " green " molecules will cross this 

 imaginary plane in opposite directions, and since this will be true 



