NUMBER AND DISTRIBUTION OF STOMATA 473 



the epidermis is variable, and so forth. Hence the amount of gaseous 

 interchange that is effected by a single stoma in one plant, may 

 require two or three stomata for its performance in another species. 

 Consequently, we must not expect to find a close correspondence 

 between the number of the stomata and the humidity of the 

 surroundings. All that can be stated in general terms is, that the 

 stomata are on the whole least numerous, where the external conditions 

 are most xerophytic. This rule may be illustrated by means of the 

 following examples, which have been selected from a more extensive 

 list compiled by Tschirch : 



Such values are naturally most strictly comparable in the case of 

 closely related species. Thus, comparatively uniform results have been 

 obtained by Spitzer with Grasses and by Ziegeler with species of 

 Carex. 



In leaves of the dorsi ventral type, the stomata are principally 

 located on the lower surface of the leaf, that is, in contact with the 

 well- ventilated spongy parenchyma. Not infrequently, however, a con- 

 siderable number of stomata occur in addition on the upper side in 

 connection with the palisade-tissue. The circumstance that a current of 

 air can traverse the whole thickness of the leaf, in such cases, is no doubt 

 an advantage from the point of view of gaseous interchange. In 

 isobilateral leaves the stomata are as a rule equally numerous on both 

 faces. The fact that the stomata of floating leaves are all located on 

 the upper side requires no explanation. 



Stomata may be uniformly distributed among the ordinary epidermal 

 cells, or they may be arranged in groups or in longitudinal series. In 

 the latter case, it is not unusual for all the members of a group or series 

 to have a common internal and external air-chamber. . 



