Il6 THE GREEN PLANT CELL 



the interchange of gases between it and the mesophyll 

 cells, and equilibrium between it and the external 

 air is constantly tending to be re-established by the 

 diffusion of gases through the stomata, into or out of 

 the leaf. Thus if the mesophyll cells absorb carbon 

 dioxide, as they do when illuminated, the pressure of 

 this gas decreases in the intercellular spaces and fresh 

 carbon dioxide from the air streams in through the 



FIG. 10. Diagram of transverse section of a leaf to show the relation 

 of the mesophyll (mes.) to the system of intercellular spaces (int.) 

 and the stomata (st.). The white arrows represent the evapora- 

 tion of water from the mesophyll cells into the intercellular 

 spaces, and its diffusion through the stomata to the outer air. 



stomata to restore the equilibrium. The oxygen pro- 

 duced in the mesophyll cells under the same conditions 

 increases the pressure of oxygen in the intercellular 

 spaces, and it escapes through the stomata into the 

 external air. The air in the intercellular spaces is 

 normally nearly saturated with water vapour, owing 

 to the constant evaporation of water from the wet 

 mesophyll cells into the air in contact with them, 

 and this water vapour is always escaping through the 



