34 



THE VEGETATIVE FUNCTIONS OF PLANTS 



them to the eye,^ in order to examine opaque objects, 

 sections of them must be cut, thin enough to be readily 

 transparent. The conditions of observation are also 

 much simplified by this means. 



Thin cross-sections of leaves, that is, sections cut at 

 right angles to the surface, are readily made with a sharp 



Fig. 27. — Cross-sections of leaves of an oak {Quercus novimexicana), 

 showing the effect of different light conditions on the internal anatomy. 

 I, from leaf growing in sunlight; 2, from leaf growing in the shade. (After 

 Clements.) 



razor. When examined with the microscope, such sections 

 disclose a structure similar to that illustrated in Fig. 27. 

 The epidermis, both upper and lower, is seen to consist 

 of a single layer of cells. The free surface of the outer 

 cell-wall is coated with a layer of a wax-like substance, 



^ Objects examined with the unaided eye are observed with light re- 

 fluted from their surface to the eye. 



