60 



PLANT PHYSIOLOGY AND ECOLOGY 



77. Structure and position of stomata. The simplest form of 

 stoma is a hole, which remains constantly open, and is surrounded 

 by peculiar epidermal cells. In all vascular plants, however, the 

 opening is regulated by two guard-cells. Below the opening regu- 

 larly occurs an air-space of variable size, the air-chamber. The 

 guard-cells var}^ considerably in different plants, but in all they 

 agree in being oblong or cylindrical cells, bent in such fashion that 

 they join broadly at the ends, but are free at the middle. The 

 outer wall of each guard-cell is thickened, and in many cases the 

 inner wall also. The wall next the opening is likewise more or 

 less thickened, while that opposite the opening, i.e., touching the 

 epidermal cell, is the thinnest. Guard-cells are almost invariably 

 filled with chloroplasts which contain starch. 



Stomata usually occur singly, scattered more or less uniformly 

 over the epidermis, but in some plants they are found in groups. 



Fig. 15. Distribution of stomata in the epidermis of an orchid, Calypso 

 borealis. The lower epidermis (1) has 36 stomata per square milli- 

 meter of surface, the upper (2) but 2 per square millimeter. 



Though commonly on a level with the epidermal cells, stomata 

 are often sunken for protection, either singly, as in the century 

 plant, or in groups, as in the oleander. Sunken stomata usually 

 possess an outer chamber or court, formed by the over-arching 

 of the epidermal cells. Their guard-cells usually exhibit one or 



