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. 1). 
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 lefgth 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- 
