32 THE VEGETATIVE FUNCTIONS OF PLANTS 
the great mullein), there are numerous more or less promi- 
nent "hairs," which are outgrowths of the epidermis, and 
readily come away with it when it is peeled off (Fig. 24). 
We notice that the veins of the leaf appear to be imbedded 
in the mesophyll, and that they lie somewhat nearer the 
lower than the upper surface. The lower surface is also 
seen to be usually of a lighter green color than the upper, 
and the lower epidermis may be peeled off more readily 
than the upper. The explanation of these facts is found 
in the microscopic structure (histology) of the mesophyll, 
soon to be studied. 
34. Histology of the Leaf-epidermis. When a small 
portion of the lower epidermis is examined under the 
FIG. 25. Lizard's tail (Saururus cernuus). Portions of leaf-epidermis; 
U, upper epidermis; L, lower epidermis; ep, epidermal cell; st, guard-cells 
of the stomata. (Camera lucida drawing.) 
microscope it is seen to be composed of larger cells, 
irregular in shape, and of smaller cells, usually somewhat 
half-moon shaped, occurring in pairs, and the pairs 
irregularly distributed at frequent intervals among the 
larger cells (Fig. 25). The latter possess no green color- 
