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 cerniiiis). 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- 



