1901.] on Some Recent Work on Diffusion. 561 



from between two parallel plates, as in fact you see they do. (See 



Fig. HO . ... 



But considerations of this kind, although of interest in showing 



the striking analogies between certain phenomena of electrostatics 

 and static diffusion, would carry me too far from my main object, and 

 I must again bring you back to the green leaf, which was the starting- 

 point of my lecture. 



If we regard the structure of the leaf from the new point of view 

 which now suggests itself, we can readily understand how it is that 

 the stomates, notwithstanding the relatively small area of the leaf 

 surface which they occupy, can drink in the atmospheric carbonic 

 acid with such rapidity. 



The finely perforated epidermis of tbe leaf, tightly stretched over 

 the interior air-spaces whose walls can absorb carbonic acid, con- 

 stitutes a multiperforate septum which is under the most favourable 

 conditions to produce an acceleration of the diffusive flow of the gas 

 into the leaf. 



The laws of gaseous diffusion through small apertures are now so 

 well understood that we can predict with certainty the particular 

 quantitative effect produced on a given diffusive flow by any screen 

 with perforations of known size and distribution providing they are 

 not within a certain number of diameters distant from each other. 

 These deductions can be verified by experimenting with small shallow 

 glass cylinders, made absorbent inside, and closed at the top with 

 very thin discs of celluloid perforated in a known manner. Such a 

 piece of apparatus may be regarded as an artificial leaf, the perforated 

 celluloid representing the epidermis with its stomates, whilst the ab- 

 sorbing solution of caustic soda acts the part of the assimilating 

 centres. 



Having obtained confidence in the accuracy of the method of 

 calculation, we can then apply the same principles to determining the 

 efficiency of the leaf stomates, when the whole system is regarded as 

 a piece of mechanism for promoting diffusion. 



In the first place, it is found experimentally that the most 

 economical arrangement of very small apertures is to have them set 

 about 8 or 10 diameters apart, for at that distance the interference 

 with each other practically ceases. This is about the distance at which 

 we generally find the stomates arranged on the underside of most leaves. 



You will remember that the amount of atmospheric carbonic acid 

 which enters an assimilating leaf in an hour, is about ■ 1 c.c. for every 

 square centimetre of leaf. Now it can be shown that for this amount 

 of gas to enter through the stomates it is only necessary for the C0 2 

 content of the air just within the leaf to be kept down to 2 • 8 parts 

 per 10,000, when that of the outer air is three parts per 10,000. 

 This very slight difference in the partial pressure within and without 

 is quite sufficient to account for all the entering C0 2 , thanks to the 

 special structure of the leaf. 



Thus all the apparent difficulties in the way of accepting the 



Vol. XYI. (No. 95.) 2 p 



