BRIGGS: THE LIVING PLANT AS A PHYSICAL SYSTEM 



103 



having their stomata all on the under side no carbon dioxide 

 was taken up by the upper surface of the leaf even in direct sun- 

 shine. On the other hand, in the case of leaves with stomata 

 on both sides, both the upper and the under side of the leaf 

 were active in assimilation. 



We have seen that according to Brown and Escombe, absorp- 

 tion of carbon dioxide by the under side of the Catalpa leaf 

 proceeds about one-half as rapidly as it would if the leaf sur- 

 face were covered with a constantly renewed film of caustic soda. 

 If, then, the absorption of carbon dioxide takes place only through 

 the stomata, the total area of which when fully opened is only 



TABLE 3 



Diffusion of Carbon Dioxide Through Circular Apertures of Different 

 Diameters, According to Brown and Escombe 



about 0.9 per cent of the area of the under side of the leaf, the 

 rate of diffusion per unit area through these openings must be 

 approximately 50 times the rate at which carbon dioxide is ab- 

 sorbed per unit area by a caustic soda solution of the same total 

 area as the leaf. 



Diffusion through perforate septa. It will at once be noted 

 that the two systems just compared differ in one important re- 

 spect. The absorbing surface presented by caustic soda solu- 

 tion is in the form of a single continuous surface. The stomatal 

 system on the other hand is made up of minute elliptical openings 

 separated by intervals from 5 to 10 times the diameter of the 



