284 HOW CROPS GROW. 
In the grasses, although the stem and leaf are distinguish 
able in shape, they are but little unlike in other external 
characters. 
In forest trees, we find the most obvious and striking 
differences between the stem and leaves. 
Green Color of Leaves.—A peculiarity most character- 
istic of the leaf, so long as it is in vigorous discharge of 
its proper vegetative activities, is the possession of a green 
color. This color is also proper in most cases to the young 
bark of the stem, a fact further indicating the connection 
between these parts, or rather demonstrating their identity 
of origin and function, for it is true, not only in the case 
of the cactuses, but also in that of all other young plants, 
that the green (young) stems perform, to some extent, the 
same offices as the leaves. : 
The loss of green color that occurs in autumn, in case 
of the foliage of our deciduous trees, or on the maturing 
of the plant in case of the cereal grains, is connected with 
the cessation of growth and death of the leaf. 
There are plants whose foliage has a red, brown, white, or other than 
agreen color during the period of active growth. Many of these are 
cultivated by florists for ornamental purposes. The cells of these color- 
ed leaves are by no means destitute of chlorophyll, as is shown by mi- 
croscopic examination, though this substance is associated with other 
coloring matters which mask its green tint. 
Structure of Leaves.—While in shape, size, modes of 
arrangement upon, and attachment to the stem, we find 
among leaves no end of diversity, there is great simplicity 
in the matter of their internal structure. 
The whole surface of the leaf, on both sides, is covered 
with epidermis, a coating, which, in many cases, may be 
readily stripped off the leaf, and consists of thick-walled 
cells, which are, for the most part, devoid of liquid con- 
tents, except when very young. (4, 4; tig. 56.) 
The accompanying figure (56) represents the appearance of a bit of 
bean-leaf as secn on a section from the upper to the lower surface and 
highly magnified. 
