Io8 THE VEGETATIVE FUNCTIONS OF PLANTS 



110. Stomata and Gaseous Exchange. — In Chapter IV 

 we described the openings or stomata in the epidermis of 

 leaves, through which water-vapor passed to the out- 

 side. We also learned in Chapter VII that the inter- 

 change of gases in photosynthesis takes place through the 

 stomata. So, also, does the exchange of gases that 



Fig. 70. — White birch {Betula popuUfolia). Portion of a branch showing 

 the prominent lenticels. 



accompanies respiration. As the oxygen within the 

 cells is consumed in respiration, more is absorbed from the 

 intercellular spaces. Thus its pressure becomes less 

 within the leaf than without, and consequently oxygen 

 enters by diffusion through the stomata. At the same 

 time the air in the intercellular spaces becomes richer 



