90 THE BIOLOGY OF FLOWERING PLANTS 



usually different on the two surfaces of the same leaf is clear 

 from examples already quoted. In the ordinary dorsi- 

 ventral leaf there are generally more stomata on the lower 

 surface ; in many cases, indeed, especially in hard and 

 leathery leaves, they are completely absent from the upper 

 surface. In erect leaves, as in grasses, and in succulent or 

 fleshy leaves the numbers tend to be more equal. In the 

 floating leaves of water plants they occur on the upper 

 surface alone. The general tendency is for the stomata to 

 occur on the surface next the best developed aerating 

 system ; this is normally the surface best protected from the 

 extreme drying influence of the sun. In the exceptional 

 case of the floating leaf stomata can function on the upper 

 surface only. The stomata are not evenly distributed over 

 the leaf surface ; they are in general more frequent along 

 the larger veins, and their density often varies from tip 

 to base. Table XX gives some idea of the range met with. 



TABLE XX 



Distribution and Number of Stomata 



De Bary gives a compilation from an extensive investiga- 

 tion by Weiss, which shows that, of 157 land plants examined, 

 12 species had fewer than 40, 42 species had 40-100, 38 

 species 100-200, 39 species 200-300, 12 species 300-400, 

 4 species over 400 stomata per sq. mm. : 675 for the olive 

 and 716 for the turnip are the largest numbers. A 

 moderately large leaf with an average density of stomata 

 may possess several millions. 



