172 



NATURE 



[June 13, 1901 



substances having a definite percentage of carbon a simple calcu- 

 lation enables us to determine the equivalent amount of carbonic 

 acid abstracted from the air. 



By such methods as this it can be shown that an actively 

 assimilating leaf, such as that of the Catalpa tree, in full day- 

 light, and under favourable conditions of temperature, can take 

 in carbonic acid from the air at the rate of about i/ioth cubic 

 centimetre per hour for each square centimetre of leaf. 



Since there are only about three volumes of carbonic acid in 

 10,000 volumes of ordinary air, this must mean that in a single 

 hour the under surface of the leaf will take in as much carbonic 

 acid as is contained in a column of air about eight feet long, and 

 having the same area of cross-section as the leaf. 



But this remarkable power of an assimilating leaf will be 

 better appreciated if we compare it with a liquid surface of a 

 strong solution of caustic alkali, which is known to have such a 

 great avidity for carbonic acid. 



We can investigate the absorptive power of such solutions for 

 the carbonic acid of the air under fixed and controllable condi- 

 tions by using a form of apparatus which I have on the table, 

 and which can be examined at the close of the lecture. It is so 

 arranged that an air current of known velocity can be drawn 

 over the surface of the absorbing solution which has a known 

 area. 



When a very low velocity of the air current has been reached 

 the amount of absorption becomes constant at ordinary tempera- 

 turesat about ^\^ c.c. of carbonic acid per square cm. of surface 

 per hour. 



So we see that a leaf, assimilating under natural conditions, 

 is taking in carbonic acid from the air more than half as fast as 

 a surface of the same area would do if it were wetted with a 

 constantly 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 conclude that it goes in more than 

 fifty times faster than it would do if the mouth of each one of 

 these minute 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 has 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 beaiing on 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 

 molecules of carbonic acid, which we will imagine for the 

 moment are distinguished from the molecules of the other 

 gases by some difference 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 k 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 for any plane surface, no matter 

 where we take it within the cylinder, there can be no change 

 in the average distribution of the " green " molecules through- 

 out the cylinder— in other words, no change in any part of the 

 cylinder in the composition of the air as regards its carbonic 

 acid content. 



But now let us imagine that the bottom of the cylinder is- 

 suddenly made capable of absorbing carbonic acid, say by the 

 introduction, without any disturbance of the air, of a little solu- 

 tion of caustic soda or caustic potash. The " green " molecules- 

 which now strike the bottom of the cylinder at all imaginable 

 angles of incidence will not all rebound as they originally did, 

 but will be to a large extent trapped in their to and fro excur- 

 sions, so that in the very first brief interval of time a very thin 

 stratum of air, parallel to and immediately above the absorbing 

 surface, will be partially freed from its "green" molecules. 



Now consider the kind of exchange of "green" molecules 

 which occurs in the next very brief interval of time between 

 this partially depleted layer at the bottom and the one immedi- 

 ately above it. The rate of exchange across the imaginary plane 

 dividing these two contiguous layers can no longer be equal and 

 opposite since the number of "green" molecules in the upper 

 stratum is greater than that in the lower. A larger number of 

 the " green " molecules must consequently pass in a given brief 

 interval of time from the higher to the lower stratum than from 

 the lower to the higher ; in other words, the balance of exchange 

 is in favour of the lower layer. This state of affairs will rapidly 

 propagate itself upwards until the mouth of the cylinder is 

 reached, and, provided the air outside the cylinder is kept of 

 the same composition and the absorptive power of the bottom of 

 the cylinder is also kept constant, these uncompensated balances 

 of exchange between the imaginary layers may be regarded as 

 constituting a steady Aow or (/;•/// of the "green" molecules 

 down the tube towards the absorbent surface. 



Although within the column there is this constant flow of car- 

 bonic acid molecules in the general direction of the axis of the 

 tube, the system as a whole may now be regarded as static 

 as long as all the conditions remain unchanged. The flow 

 is, then, strictly analogous to the " flow " of heat in a bar of 

 metal which is kept with its two ends at a uniform difl'erence of 

 temperature, or to the flow of electricity in a conductor betweer> 

 two regions maintained at a constant difference of potential ; and 

 static diffusion admits of precisely the same simple mathematical 

 treatment as these phenomena of conduction of heat or electricity 

 when we come to its quantitative study. 



In such an imaginary experiment as we have been considering 

 it is clear that the amount of carbonic acid in the air of the 

 cylinder must vary uniformly from a maximum at the top of the 

 cylinder to a vanishing point at the bottom, so that if the CO, 

 really had the green colour which, for purposes of argument, we 

 have attributed to it, the depth of colour of the air column would 

 uniformly diminish from top to bottom. 



This can be illustrated by the diffusion of a coloured coppej 

 salt down a gelatine column. If this column were cut off just 

 where the colour ceases to be perceptible, and the cut end were 

 immersed in water to carry off the diffusing salt as fast as it 

 came through the column, then if the upper end of the column 

 remained iii contact with the coloured copper solution we should 

 ' ultimately get a constant steady flow of the salt down the 

 column. 



Under these conditions it can be readily shown, both experi- 

 mentally and theoretically, that the actual amount of substance 

 diffusing down the column in a given time will, in the first 

 place, be directly proportional to the difference in the concen- 

 tration of the diffusing substance at the two ends of the column ; 

 it will also be directly proportional to the area of cross-section 

 of the column, but inversely proportional to its length. 



The fact which for the moment I wish you to bear in mmd is 

 that, all other things being the same, the amount of diffusion 

 down a column of this kind varies directly as the area of the 

 cross-section of the column. ^ . , , 



This is roughly illustrated by these two cyhndrical columns or 

 gelatine of different diameters, down which a coloured solutiOQ 

 has been diffusing for equal times. 



NO. 1650, VOL. 64] 



