124 INTAKE AND DISTRIBUTION OF GASES 



The cutinized and suberized walls of epidermis and cork, 

 which, as we have learned, in Chapter IV, are necessary to 

 keep plants from drying up, also retard the inflow and outflow 

 of gases, and this fact has made necessary the passage ways 

 through the epidermis and cork known as stomata and lenticels. 



While every living cell in the plant body must have free oxygen 

 for its respiration, all cells which make the food of the plant 

 from carbon dioxide and water, namely the green cells, and 

 particularly those in the leaves, must have carbon dioxide brought 

 to them. There are times in the spring and summer, during 

 rapid growth or the storage of reserve foods in seeds, tubers, 

 etc., when carbon dioxide must be absorbed in large quantities. 

 But this gas exists in the atmosphere in very dilute solution, 

 namely, there are between three and four parts of it in 10,000 

 parts of atmosphere, and every facility must be offered for its 

 entrance and distribution. Accordingly we find stomatal open- 

 ings through the epidermis of leaves, and relatively large, con- 

 tinuous, open ways between the food manufacturing cells within 

 the leaves. 



Doubtless the need of the absorption and distribution of 

 oxygen and carbon dioxide is the chief reason for the existence 

 of an aerating system composed of openings and passage ways; 

 but vapor of water is excreted into the same channels and through 

 them is thrown off into the air. How much this is merely inci- 

 dent to the fact that the passage ways are there, and how much 

 the well-being of the plant demands this excretion of water 

 has not been definitely determined. There are times of great 

 turgidity of the tissues when water filtrates into the intercellular 

 spaces; but this water soon evaporates through the stomata and 

 lenticels,- leaving the spaces again open. 



The intercellular spaces, then, serve three main purposes: 

 They provide for the admission and distribution of oxygen used 

 in respiration, and of carbon dioxide employed in food-making, 

 and they furnish a way for the elimination of water vapor. Other 

 kinds of intercellular spaces and other uses for them will be 

 spoken of in Chapter XII. 



