154 HISTOLOGY OF MEDICINAL PLANTS 



of the guard cells is rounded or has a curved outline; but in a 

 few cases the guard cells have angled outlines. 



The arrangement of the surrounding cells of the stoma 

 is one of the most important characteristics of the different 

 leaves. As a rule the number of surrounding cells about a 

 stoma is constant for a given species. In senna leaves (Plate 

 54, Fig. 2) there are normally two surrounding cells about 

 each guard cell, while in coca there are four (Plate 55, Fig. i). 

 In senna the long diameter of the surrounding cells is parallel to 

 the long diameter of the guard cells ; but in coca the long diameter 

 of two surrounding cells is at right angles to the long diameter of 

 the guard cells, while two cells are parallel to the long diameter 

 of the guard cells. 



In most leaves there are more than two cells around the 

 guard cells. 



The form and size of the surrounding cells must always be 

 considered. In most leaves they are variable in size and form. 



Guard cells occur first, even with the surface of the leaf (Plate 

 56, Fig. A); secondly, above the surface of the leaf (Plate 56, 

 Fig. B) ; and, thirdly, below the surface of the leaf. (Plate 56, 

 Fig. C). Only one of the above types occurs in a given species 

 of plant. That is, plants with stomata above the surface of the 

 leaf do not have stomata on a level with or below the leaf 

 surface. 



The number of stomata on a given surface of a different leaf 

 varies considerably. 



In many of the medicinal leaves stomata occur only on the 

 under surface of the leaf. In other leaves stomata occur on both 

 surfaces of the leaf; but in such cases there are a greater number 

 on the under surface. 



In certain leaves the long diameter of the guard cells is 

 parallel to the length of the leaf; in other cases the long diameter 

 of the stoma is arranged at right angles to the length of the leaf. 



In other leaves the arrangement is still more irregular, the 

 guard cells assuming all sorts of positions in relation to the 

 length of the leaf. 



The relation of the stoma to surrounding cells is best shown 

 in cross-sections of the leaf. In powders the relationship of 

 the stoma to the surrounding cells is, however, readily ascer- 



