90 Plant Physiology 



is a fact of peculiar interest. Furthermore, it has been 

 calculated that the capacity of the stomata in the sun- 

 flower, for example, is about six times as great as any 

 observed transpiration ; that is, the stomata only one 

 sixth open would be sufficient to accommodate the most 

 rapid loss of water which has been observed. 



The stomata exhibit a considerable range in size, but 

 according to Eckerson the average approximates 18 x 6 /a. 

 This minute size is scarcely appreciated until one com- 

 pares it with some visible perforation, such as a needle- 

 prick made with the smallest sewing needle, which is rela- 

 tively enormous, measuring about 600 /z in diameter. 

 Nevertheless, the total maximal stomatal opening of an 

 average leaf is approximately one nineteenth of the 

 surface. 



53. Distribution of stomata. - - While stomata may 

 occur in the epidermis of any plant organs, they are com- 

 monly confined to the aerial surfaces, and especially to 

 the leaves, or to organs performing the functions of leaves. 

 As a general rule, in fact, it may be said that the under 

 surfaces of the leaves are the situations most important 

 with respect to stomatal occurrence. Eckerson has found 

 that only about two fifths of the common greenhouse 

 plants possess stomata on the upper surfaces. Weiss 

 and others have collected considerable data showing the 

 relative abundance of the stomata upon the different sur- 

 faces of dorsi-ventral leaves, from which the following 

 examples may be suggestive : 



