CHAPTER XIII 

 THE STOMATAL MECHANISM 



The most important physiological fact about the stomates is that they 

 are sometimes open and sometimes closed. When open they serve as the 

 principal pathways through which gaseous exchanges take place between the 

 intercellular spaces of the leaf and the surrounding atmosphere. When closed 

 all gaseous exchanges between a leaf and its environment are greatly retarded. 

 The gases of greatest physiological importance which enter or depart from 

 a leaf principally through the stomates are oxygen, carbon dioxide and water- 

 vapor. The movement of gases through the stomates in either direction is 

 primarily a diffusion phenomenon, although under certain conditions, as for 

 example when the intercellular spaces are alternately compressed and expanded 

 by bending of the leaf in a high wind, alternate outward and inward mass 

 movements of gases may occur. Although the stomates are the principal 

 portals through which entry and escape of gases takes place, the fact should 

 not be overlooked that at least small amounts of these three gases pass directly 

 through the epidermis and cuticular layers of all leaves. In submerged aquatics 

 all gaseous exchanges between the plant and its environment occur through 

 the epidermis. 



Structure of the Stomates. — The stomates or stomata (singular stomate 

 or stoma) are minute pores which occur in the epidermis of plants. They 

 are surrounded by two specialized epidermal cells known as the guard cells. 

 Stomates may occur on any part of a plant except the roots, but in most 

 species are most abundant upon the leaves. The size of the stomatal pore 

 varies in most plants depending upon the turgidity of the guard cells and 

 often, especially at night, it is entirely closed. In Fig. 42 are depicted sur- 

 face and cross-sectional views of several of the commoner types of stomates. 

 The structure of stomatal apparatus shows marked variations in detail in 

 different species of plants, but the essential feature of a pore between two 

 modified epidermal cells is common to all species of vascular plants. Guard 

 cells which are roughly kidney or bean-shaped as seen in surface view are 

 typical of more species of plants than any other kind (Fig. 42, A). In some 

 species the epidermal cells bordering on the guard cells are different in con- 



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