120 TEXTBOOK OF BOTANY 



Leaves and photosynthesis. In most land plants the bulk of photosyn- 

 thesis occurs in the leaves. We ha\'e already seen that leaves of various 

 species of plants may differ greatly in several ways, such as the amount 

 of cutin on the epidermis, the number of stomates per square centimeter 

 of surface, the thickness of the leaf, the compactness of the chlorenchyma 

 in relation to the number and size of intercellular spaces, and the conse- 

 quent exposure of the mesophyll cell walls to the internal atmosphere of 

 the leaf (Chapter X). All these structures may influence the entrance 

 of carbon dioxide into the cells of the chlorenchyma and thus indirectly 

 influence the rate of photosynthesis. 



Moreover, the environment in which a leaf grows may greatly affect 

 the processes underlying the development of these structures (Chapter 

 IX). Hence plants of the same species that have grown in different 

 environments may have different rates of photosynthesis. Experiments 

 have shown that certain plants growing in a relativeh' dry atmosphere 

 may make more sugar during their lifetime than plants of the same 

 species growing in a very humid atmosphere. A part, or all, of this dif- 

 ference may depend upon differences in the development of the leaf 

 structure in dry and moist air, particularly the structure of the chloren- 

 chyma and stomates. 



The mesophyll. The amount of mesophyll wall surface exposed to the 

 internal atmosphere of the leaf is considered to be one of the important 

 factors in photosynthesis. Through it most of the CO- enters the cells 

 of the chlorenchyma. Some CO2 may enter the leaf directly through the 

 epidermal cells, but entrance in this manner may be gready impeded 

 by the thickness of the outer walls of the epidermal cells and by the 

 presence of cutin. Moreover, the area of the exposed mesophyll surface 

 is usually many times greater than the total area of the epidermis. From 

 measurements made on several different kinds of leaves it was found 

 that the area of this inner exposed surface may be from 9 to 30 times 

 greater than that of the epidermis. The data from numerous tests indi- 

 cate that in leaves of land plants the COj enters the cells of the chloren- 

 chyma largely from the internal atmosphere of the leaf. 



The epidermis. The chief structures in the epidermis that influence the 

 rate of photosynthesis are the guard cells enclosing the stomates through 

 which CO2 enters the intercellular spaces of the chlorenchyma ( Chapter 

 VIII ) . When one begins to consider stomates in relation to the entrance 

 of CO2, several complicated problems in physics soon arise. These prob- 

 lems will not be discussed here, but a few facts will be stated briefly. 



