June 20, 1901] 



NA TURE 



195 



correct diffusion, through apertures having their diameters in the 

 ratio of 2 to I ought to give the same series of figures. You 

 see from the photograph of an actual experiment given in Fig. 9 

 that this supposition is correct. 



In Fig. 10 are given the calculated lines of force at the edges 

 uf two parallel plates, one of which is insulated and electrified, 

 the other connected with the earth. These ought to correspond 

 in shape to the equi-density lines of a substance undergoing steady 

 diffusion from between two parallel plates, as, in fact, you see they 

 do. (See Fig. II). 



for every square ccntiuielre of leaf. Now it can b^ siiown that 

 for this amount of gas to enter through the stomates it is only 

 necessary for the CO^ 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 

 3 parts per 10,000. This very slight difference in the partial 

 pressure within and without is quite .sufficient to account for all 

 the entering CO^, thanks to the special structure of the leaf. 

 Thus all the apparent difticulties in the way of accep'ing the 



But considerations of this kind, although of interest in showing 

 the striking analogies between certain phenomena of electro- 

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

 subject, 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 

 the leaf, tightly stretched over the 

 interior air-spaces, whose walls can 

 absorb carbonic acid, constitutes a 

 multiperforate septum which is under 

 the most favourable conditions to pro- 

 duce an acceleration of the diffusive 

 flow of the gas into the leaf. 



The laws of gaseous diffusion through 

 small apertures are now so well under- 

 stood that we can predict with certainty 

 the particular quantitative effect pro- 

 duced 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 deduc- 

 tions can then be verified by experi- 

 menting 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 absorbing 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 inter- 

 ference 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 'I c.c. 

 NO. 165 1, VOL. 64] 



minute stomates as the sole pathways of gaieou-i exchange in 

 the leaf entirely disappear when the leaf is studied in this new 

 light, and it becomes evident that the adjustment of the 

 mechanism of the leaf to the physical properties of its surround- 

 ing m;dium is far more perfect than has been hitherto suspected. 

 The leaves of plants have, in fact, proved to be better physicists 



than we are, .sijice their structure bears the impress ul response to 

 certain properties of gases of which we have hitherto been ignorant. 



This is by no means the first occasion on which the plant has 

 given us a lead in physics. The theory of dilute solutions, for- 

 mulated by van 't Hoff, and indicating that the laws of Boyle 

 and of Avogadro are as applicable to dilute solutions as they 

 are to gases, had its origin in the observations of De Vries and 

 of Pfeffer on the plasmolysis of living cells and the properties 

 of natural semi-permeable membranes. 



Nor can we doubt that there are many more such instances 

 which only await detection, and we may reasonably hope that 

 the boundaries of physics and of chemistry will be materially 

 enlarged in unexpected directions if we pay due regard to the 

 whispered hints and slender clues which are on all sides given 

 by the living world of Nature. 



