The Path of Gaseous Exchange. 



57 



Injecting the leaf with water has a similar influence on altering 

 the ratio of COa-intake by the two surfaces as coating the lower 

 surface with vaseline. By such injection tlie intercellular spaces 

 are filled with water and diffusion of carbon dioxide can then only 

 take place in solution ; hence the stomatal surface no longer 

 possesses such an advantage over the upper surface in regard to 

 the passage of carbon dioxide through it. The following table 

 shows the relative amounts of carbon dioxide respired from the two 

 surfaces of leaves under various conditions. 



Table VII. 



Blackman thus comes to the conclusion that the epidermis 

 with its cuticle is slightly permeable to carbon dioxide, but that 

 under normal conditions, by far the greater part of gaseous 

 exchange takes place through the stomata. Under artificial 

 conditions, such as waterlogging the intercellular spaces or 

 blocking the stomata, the passage of carbon dioxide through the 

 cuticle, though not actually greater, may become of relatively more 

 importance. 



An extended series of experiments bearing on the same matter 

 of the path of carbon dioxide into the leaf has been made by 

 Stahl (1894). His method consisted in artificially blocking the 

 stomata on parts of the leaf and showing that after exposure to 

 light, starch formation is limited to the regions of the leaf where 

 the stomata were unblocked. Some similar experiments were 

 madeindependently by Blackman wiio confirms Stahl's observations. 



The experiments of Blackman, Stahl and Brown and Escombe 

 appear to show conclusively that the path of diffusion of carbon 

 dioxide into the leaf is mainly or entirely through the stomata. 

 But there are three facts which rendered difficult the acceptance of 

 this evidence alone. These facts are (1) the large amount of carbon 

 dioxide absorbed by a leaf during active assimilation ; (2) the low 

 partial pressure of carbon dioxide in the atmosphere — the carbon 

 dioxide only amounts to about 3 parts per 10,000 of the atmosphere ; 

 and (3) the very small fraction of the leaf surface occupied by the 

 stomata. 



