FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 37 
support and conduction are very perfect (see Fig. 27). It 
is also clear that the green substance thus supported and 
supplied with material is the important part of the leaf, the 
part that demands the light-relation. Study the various 
plans of the vein systems in Figs. 3, 9, 13, 18, 19, 20, 21, 
25, 26, 51, 70, 76, 82, 83, 92, 161. 
FIG. 27. A plant (Fittonia) whose leaves show a network of veins, and also an adjust- 
ment to one another to form a mosaic. 
30. Epidermis. If a thick leaf be taken, such as that 
of a hyacinth, it will be found possible to peel off from 
its surface a delicate transparent skin (epidermis). This 
epidermis completely covers the leaf, and generally shows 
no green color. It is a protective covering, but at the same 
time it must not completely shut off the green substance 
beneath from the outside. It is found, therefore, that 
three important parts of an ordinary foliage leaf are : (1) 
