194 ELEMENTARY STUDIES IN BOTANY 



stomata are to permit communication with the air without 

 permitting an excessive loss of water, their distribution be- 

 comes a matter of course. For example, in horizontal 

 leaves, the stomata are chiefly and sometimes exclusively 

 on the under, more shaded, surface, a surface less dangerous 

 to open to the drying air than the upper surface ; on erect 

 leaves (as iris), in which both surfaces are exposed alike to 

 the light, the stomata are equally distributed ; in floating 

 leaves (as water-lilies) the stomata are naturally all on the 

 upper surface ; while in submerged leaves there are no 

 stomata at all. Stomata are not peculiar to the epi- 

 dermis of leaves, for they are found in the epidermis of any 

 green part, as young stems, fruit, etc., and even on the petals 

 of flowers. 



(2) MesophylL Between the two epidermal layers is 

 the mass of green tissue making up the body of the leaf 

 (Fig. 157), and called mesophyll (" middle region of leaf "). 

 Of course it is these cells that contain the chloroplasts and 

 are the working cells that the epidermis protects against 

 the drying effect of air, and to which the stomata permit 

 access of air. In horizontal leaves, the cells of the meso- 

 phyll usually are arranged differently in the upper and lower 

 regions of the leaf. Those in contact with the upper epider- 

 mis are elongated at right angles to the surface of the leaf 

 and stand in close contact, being the palisade cells (Fig. 157). 

 The mesophyll cells beneath the palisade layer are irreg- 

 ular in form, and so loosely arranged as to leave air-spaces 

 between the cells, the whole region forming the spongy tissue 

 (Fig. 157). The air-spaces communicate with one another, 

 forming a labyrinth of air-passages throughout the spongy 

 mesophyll. It is into this system of air-passages that the 

 stomata open (Fig. 157), so that all of the green cells are 

 put in contact with the air. The picture of a l^af^a food- 

 manufacturing machine, therefore, is that of/^an internal 

 and moist atmosphere bathing the working cells and com- 



