284 HOW CROPS GEOW. 



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 LeaveSi — ^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 tliat of all other young plants, 

 that the green (young) stems perform, to some extent, the 

 same ofiices 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 lias a red, brown, white, or other than 

 a green 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. — WTiile 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. 



Tlie whole surface of the leaf, on both sides, is covered 

 with epidermis, a coating, which, in many cases, may be 

 readily stripped oif the leaf, and consists of thick- walled 

 cells, which are, for the most part, devoid of liquid con- 

 tents, except when very young. (E, E, fig. 56.) 



The accompanying figure (56) represents the appearance of a bit of 

 bean-leaf as seen on a section from the upper to the lower surface and 

 highly magnified. 



Digitized by Microsoft® 



