230 DIFFUSION THROUGH STOMATA 



Then x = &AP. 

 and y = &AP'. 



Of these quantities A is common, P is constant, about *0003 of an at- 

 mosphere, but P' depends upon the ratio of x to the rapidity with 

 which the carbon dioxide is absorbed. This last will be proportional 

 to the area of the absorbing surface say S. 



.. =_ 



Now the rate of diffusion is 



x - y = &AP - 



= &AP - &A ( 



V B / 



Dividing by kP it is seen that the rate of diffusion is proportional to 



A - A 2 

 S 



or, per unit area, to 



B* 



Hence the smaller the value of the area of the aperture the greater is 

 the amount of diffusion per unit area. The essential point in connec- 

 tion with this phenomenon is that by means of small apertures it is 

 possible to have, on the one side, air containing practically its full 

 amount of carbon dioxide, while on the other, the inside, the air is 

 kept practically devoid of that gas ; consequently very little diffuses 

 outwards, provided the aperture be very small compared with the area 

 of the absorbing surface. 1 



In the cases of the leaves of two plants, Catalpa bignioides and 

 Helianthus annuus, Brown and Escombe made approximate measure- 

 ments of the superficial area of the spongy absorptive surfaces of the 

 cells of the parenchyma and of the area of the stomata opening into 

 the space. They found a ratio in the case of the sunflower of about 

 212 : 1, in the case of catalpa of 1159 : 1. 



In the case of Helianthus the maximum rate of absorption of 

 carbon dioxide by direct measurement was about 0*134 cc. per square 

 centimetre of leaf surface per hour. This, according to Brown and 

 Morris, would result if the partial pressure of the carbon dioxide 

 within the intercellular space were reduced by only about 6 per cent. 

 If the absorption of carbon dioxide were perfect and able to keep the 

 partial pressure at practically nil the amount of absorption of a heli- 

 anthus leaf should be about 2 or 2*5 cc. carbon dioxide per square 

 centimetre per hour if the stomata be fully opened, or the area of the 

 openings might be reduced to T V of their maximum and yet allow of 

 the maximum observed absorption. 



1 This explanation, based on the kinetic theory of gases, appears to the author 

 to be clearer and more in accordance with what he believes to be the true mechan- 

 ism of the phenomenon than the more elaborate and more mathematical conception 

 described by Brown and Escombe, in which the process of diffusion is pictured as 

 analogous to a flux or flow of carbon dioxide through the aperture. 



