THE VEGETATIVE ORGANS OF PLANTS. 285 
a 
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 a, fig. 56, is seen the cross 
section of a vein in the bean-leaf. 
The epidermis, while often smooth, is 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 
