THE VEGETATIVE ORGANS OF PLANTS. 



285 



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 

 numerous and large interspaces. 



The interspaces among the leaf-cells are occupied with air, 

 which is also, in most cases, the only con- 

 tent of the epidermal cells. The active 

 cells of the leaf contain some or all of the 

 various proximate principles which have 

 been already noticed, and in addition 

 the coloring matter of vegetation, the 

 so-called chlorophyll, or leaf-green, p. 

 109. Under the microscope, this sub- 

 stance is commonly seen in the form 

 of minute grains attached to the walls 

 of the cells, as in fig. 56, or coating 

 starch granules, 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 bundles or fibers of the stem, of 

 which they are branches. At , fig. 50, is seen the cross 

 section of a vein in the bean-leaf. 



The epidermis, while often smooth, h frequently beset 

 with hairs or glands, as seen in the figure. These are va- 

 riously shaped cells, sometimes empty, sometimes, as in 

 the nettle, filled with an acid liquid. Their office is little 

 understood. 



Leaf-Pores, The epidermis is further provided with a 

 vast number of curious " breathing pores," or stomata, by 

 means of which the intercellular spaces in the interior of 

 the leaf may be brought into direct communication with 

 the outer atmosphere. Each of these stomata consists 



Fi-. 50 



