LEAVES. 47 



SECTION III. 



POWDERED LEAVES. 



The chief diagnostic characters of powdered leaves are to be 

 found in: (a) The shape and appearance of the cells of the 

 epidermis and mesophyll ; (&) the stomata, their distribution and 

 relation to the surrounding cells ; (c) the presence or absence 

 of secretory tissue ; its nature if present ; (d) the presence or 

 absence of simple or glandular hairs and their nature, if present ; 

 (e) the presence or absence of crystals ; (f) the presence or 

 absence of pericyclic fibres ; (g) the elements of the midrib. 



Epidermis. 



The epidermis presents certain anatomical features that are 

 constant in all the leaves furnished by the same species growing 

 under normal conditions. It is covered by a cuticle that is 

 usually thin in the leaves of herbaceous plants (hemlock), but 

 thicker in t>he leaves of shrubs (jaborandi). The cuticle is 

 for the most part smooth, but it is sometimes furnished with 

 more or less prominent ridges (belladonna), or even with pro- 

 tuberances (coca). 



In transverse section the cells of the epidermis usually exhibit 

 a rectangular outline, and this is especially the case with those 

 from the interneural spaces, those that lie above or below the 

 midrib being rounded at the angles 'and therefore more or less 

 oval. The thickness of the outer wall and of the cuticle and 

 cuticular layers should be observed, as well as the relative posi- 

 tion of the stomata. In some leaves these are depressed below 

 the surface of the epidermis, in others they are raised above it. 

 In surface view the shape of the epidermal cells is extremely 

 variable, but at is constant for the same species. Sometimes they 

 are polygonal with straight walls (orange), but in very many 

 leaves the walls are more or less undulating (tea), or even so 

 sinuous as to present no regularity in shape or direction (fox- 

 glove). In thick, coriaceous leaves the epidermal cells exhibit 

 the same shape over the entire lamina, except over the midrib, 

 where they are commonly fairly regular and quadrilateral. In 

 thin leaves, on the other hand, the epidermal cells over the 

 midrib and lateral veins are almost always rectangular and 

 elongated in the direction in which the veins run. 



