XI. THE PLANT 229 



chlorophyll-containing cells, considerable advances have recently been 

 made. Blackman in 1895 l described experiments by which he proved 

 that carbon dioxide found its way into (in assimilation) and out of (in 

 respiration) the leaves almost exclusively by the stomata, and not, as 

 was generally believed, by diffusion through the cuticle and epider- 

 mis. When the rate at which carbon dioxide is absorbed by a vigor- 

 ously growing leaf in sunlight is taken into account, and also the very 

 limited area of the stomatal slits, it is difficult to realise how the ne- 

 cessary interchange through these slits can take place. Brown and 

 Escombe 2 found that a leaf of Catalpa bignioides can absorb from 

 ordinary air, containing three parts per 10,000 of carbon dioxide, 

 about -07 cc. (N.T.P.) of carbon dioxide per square centimetre of 

 leaf surface, per hour. On each square centimetre there were 14,500 

 stomata, and each stoma, when fully open, had an area of '0000618 

 square millimetre. Consequently the united area of the stomatal open- 

 ings only amounted to 0'9 per cent of the whole surface. Hence, if all the 

 absorption took place by diffusion through these openings, diffusion of 

 carbon dioxide through them must have taken place at the rate of 

 about 7 '77 cc. per square centimetre, per hour. With strong caustic 

 soda solution they found that the rate of absorption of carbon di- 

 oxide from normal air by a free surface varied from 0*12 cc. to 0*177 

 cc. per square centimetre, per hour. So that a leaf of catalpa in 

 sunlight, absorbs carbon dioxide at about half the speed at which it 

 would if covered with a continually renewed film of caustic soda solu- 

 tion, and if all absorption occurs through the stomata the carbon di- 

 oxide must move about fifty times as fast through the openings as it 

 would if they were filled with a strong solution of caustic soda. 



Brown and Escombe have shown, however, that, when an absorb- 

 ent surface is covered with a diaphragm placed some distance above 

 it, the rate of diffusion of a gas from the outside air into the absorbing 

 chamber, per unit area, increases enormously with a diminution of size 

 of the aperture. This fact is understood by applying the kinetic 

 theory of gases to the problem. The chance of any given molecule of 

 carbon dioxide moving by virtue of its kinetic motion into the cell is 

 proportional to the area of the opening ; but once within the cell its 

 chance of moving out again is less and less as the size of the aperture 

 diminishes. Now the rate of diffusion through an aperture is the 

 difference between the number of molecules which move in and out in 

 a given time. 



The number of molecules, so long as the temperature remains 

 constant, moving inwards is solely dependent upon 



(1) The area of the aperture say A. 



(2) The partial pressure of the carbon dioxide in the atmosphere 

 outside say P. 



The number of molecules moving outwards similarly depends upon 



(1) The area of the aperture A. 



(2) The partial pressure of the carbon dioxide in the chamber say P'. 

 Let x number of molecules entering in one second and y = 



number leaving in one second. 



1 Phil. Trans., vol. 186 (1895), 485. 2 Ibid., vol. 193 (1900), 232. 



