124 Ingvar Jorgensen and Walter Stiles. 
Injecting the leaf with water has a similar influence on altering 
the ratio of C0 2 -intake by the two surfaces as coating the lower 
surface with vaseline. By such injection the 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. 
Condition of Leaf. 
Relative output of C0 2 . 
Upper surface. 
Lower surface. 
Normal leaf 
1 
39 
Injected ,, 
1 
10 
Vaselined under surface 
1 
3 
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 
made independently by Blackman who 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 surfaee occupied by the 
stomata. 
