34 



THE VEGETATIVE ETJNCTIONS 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 sec- 

 tions 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 sub- 



' Objects examined with the unaided eye are observed with light rc- 

 flecled from their surface to the eye. 



