28 



General Botany 



The mesophyll tissue. The mesophyll tissue is composed of 

 the soft, thin-walled cells that He among the veins in the interior 

 of the leaf. In most leaves there are beneath the upper epidermis 

 one or more palisade layers, which are composed of elongated cells 

 standing close together, as is shown in Figure 1 1 . The remainder 

 of the mesophyll tissue is made up of ovoid or irregularly shaped 

 cells joined quite loosely, so that air spaces are left between them. 

 In fact, a much larger part of the surfaces of these cells than of 

 other cells is in contact with air spaces. The air spaces within 

 the leaf are continuous, and through them the oxygen and carbon 

 dioxide of the atmosphere can reach every cell in the leaf. We 

 shall see later that the differences in the epidermal and mesophyll 

 cells, and in the way they are arranged, are definitely related to 

 the different processes carried on by each of them. 



The mesophyll in many plants contains other cells in addition 

 to chlorenchyma. These additional cells are filled with water, 

 and sometimes form a compact layer between the epidermis and 

 the chlorenchyma, as in the begonia; they may also occur in 

 long lines along the veins, as in the corn plant. It is the loss of 

 water from these colorless water-storage cells that causes leaves 



Fig. 1 8. Illustrating terms used in describing the apexes and bases of leaves: A, acute; 

 B, acuminate; C, obtuse; D, truncate; E, aristate; F, mucronate; G, emarginate; 

 H, rounded ; /, cordate ; J, obliquely cordate ; K, acute ; L, acuminate ; M, sagittate ; 

 N, hastate 



