100 DISTRIBUTION OF STOMATA [CH. 



As regards numbers, while species of Sempervivum, 

 Sedum, &c, may have as few as 10 to 20 per sq. mm., the 

 Cabbage has on the same area 700 or more beneath and 

 400 or more above. 



There are 600 per sq. mm. in the Olive, 400 in the 

 Oak, and about 100 to 300 on most ordinary leaves. 

 Weiss found in 147 species of land plants examined, less 

 than 40 per sq. mm. in twelve species, 40 100 in forty- 

 two species, 100200 in thirty-eight, 200300 in thirty- 

 nine, 300 400 in twelve, 550 in one, and 600 700 in 

 three. In most of our trees and shrubs, 40 300 per 

 sq. mm. have been estimated. 



Much variety occurs also in the mode of distribution 

 of stomata on the surface of the leaf. In the typical 

 cases they are equally distributed all over, except over 

 the veins. But in Grasses, Pines, Firs, &c, they are in 

 definite longitudinal rows, and these stomatal lines are 

 very evident as silvery streaks; or they may be in isolated 

 groups, each with numerous stomata (e.g. Saxifraga sar- 

 mentosa, S. japonica, &c.) ; or in pairs, fours or sixes 

 grouped over a common respiratory cavity that is to say, 

 a large intercellular space in the mesophyll e.g. many 

 Begonias. In Nerium oleander, &c, groups of stomata 

 are found lining the sides of a deep depression in the 

 surface of the leaf, each depression being lined by 

 numerous hairs ; and in most cases where the edges of 

 the leaves are rolled together, or where furrows and de- 

 pressions occur, the stomata are found only in the hollows, 

 and often protected by hairs. Even more curious ar- 

 rangements occur, as in the small Orchid Bolbophyllunt, 

 where the stomata are confined to the interior of hollow 

 tubers which bear the minute leaves. In these cases 

 of depressed and protected stomata we have no doubt 

 adaptations to prevent the blocking of the stomata by 



