308 



HOW CROPS GROW. 



Tliere are many leafy plants cultivated for ornamental purposes 

 with more or less brown, red, yellow, white, or variegated foliage, 

 which are hy no means destitute of chlorophyl, as is shown by mlcro- 

 siopio 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 sim- 

 plicity in the matter of their internal structure. 



The whole surface of the leaf, on both sides, is cov- 

 ered 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 liq- 

 uid contents, except when very young. {E, E, Fig. 66.) 



rig. 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. 



Below the upper epidermis, there often occur one or 

 more layers of oblong cells, whose sides are in close con- 

 tact, and which are arranged endwise, with reference to 

 the flat of the leaf. Below these, down to the lower epi- 

 dermis, for one-half to three-quarters of the thickness of 

 the leaf, the cells are commonly spherical or irregular in 

 figure and arrangement, and more loosely disposed, with 

 nu-merous and large interspaces. 



The interspaces among the leaf-cells are occupied with 

 air, which is also, in most cases, the 

 only content of the epidermal cells. 

 The interior cells of the leaf are filled 

 with sap and contain the chloropKyl- 

 ■ granules. Under the microscope, these 

 are commonly seen attached to the walls 

 ' of the cells, as in Fig. 56, or coating 

 grains of starch, or else floating free in 

 the cell-sap. 



The structure of the veins or ribs of 



the leaf is similar to that of the vascular 



Fig. 56 bundles of the stem, of which they are 



branches. At a. Fig. 56, is seen the cross section of a 



vein in the bean-leaf. 



