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TEXTBOOK OF BOTANY 



The stomates. In describing the epidemiis the paired guard cells 

 which surround the stomate were mentioned. These are highly spe- 

 cialized cells formed by subdivision of epidermal cells; unlike the or- 

 dinary epidermal cells, they contain green plastids (Fig. 36). In tree 

 leaves the guard cells do not separate until the leaf has attained a fourth 

 or a third of its size. In many leaves they are found exclusively, or 

 mostly, in tlie lower epidermis. Less frequently they are most abundant 

 on the upper surface; they rarely occur in equal numbers on both sides 



Fig. 36. Stomates in relation to guard cells, subsidiary cells, and ordinary epidermal 

 cells. Lower epidermis of (A) Boston fern, (B) Zehrina, and (C) corn. 



of the leaf. Stomatal openings are so minute that the area of an average 

 pinhole may be equivalent to that of 2000 to 2500 stomates. On many 

 common leaves, however, there are from 100 to 600 stomates per square 

 millimeter of leaf surface, and when they are open their total area may 

 be equal to nearly 1 per cent of the lower leaf surface. 



The most remarkable thing about stomates is that they are opened by 

 the swelling and arching of the guard cells, and closed by the shrinking 

 and straightening of these cells. When a plant is moved from darkness 

 to light, the guard cells are affected by light and their internal pressure 

 increases, causing them to separate and open the stomate. On a warm 

 day this opening may take place in about 15 to 30 minutes. Thus in the 

 summertime stomates open after sunrise and remain open for two or 

 more hours, depending on conditions discussed in Chapter XXIII. The 

 closing of the stomates depends on a number of factors, mostly internal. 



