156 



BIOLOGICAL CHEMISTRY 



guarded by cells which open or close according to the 

 wetness or dryness of the atmosphere. These openings 

 regulate the escape of moisture by evaporation, and they also 

 influence the gas exchanges. Lenticels are minute openings 

 on stems. 



In the leaf are air spaces into which the stomata open. 

 Gas exchange takes place between the cells of the leaf and the 

 air spaces. The air spaces exchange gases with the external 

 air through the stomata. 



FIG. 29. Drawing of under 

 surface of leaf. 



A, opening leading into air spaces 

 of leaf between, B, the guard cells of 

 the stoma. 



Slightly modified from" Vegetable 

 Physiology." J. R. Green (Churchill) . 



FIG. 30. Drawing of lenticel show- 

 ing the manner in which gaseous 

 exchange can take place between 

 stems and the surrounding atmo- 

 spheres. 



Copied from " VegetaJile Physiology," 



J.R. Green (Churchill). 



Diffusion through the openings in septa has been studied by 

 Brown and Escombe,* who have found that in plants the 

 stomata are more than sufficient for the exchange of gases 

 by diffusion. Under natural conditions leaves and branches 

 are always moving in air currents. These movements cause 

 deformation of the air spaces in them so that the air in the 

 spaces is kept moving, thus the mixing of gases in the spaces 

 and movements of gases through the stomata and lenticels 

 are promoted. 



Water plants sway in the water, thus their surfaces are 

 brought into contact with fresh portions of water. The 

 exchange of gases is facilitated by these movements. Some 

 water plants contain air sacs so that gas exchange can take 

 place into the air sacs as well as into the surrounding water. 



Aquatic animals are furnished with gills and by movements 

 of the animal water currents are forced over the gill fringes 



* H. T. Brown and F. Escombe, Phil. Trans., 1900, B. 193, p. 223. 



