98 VEGETABLE HISTOLOGY. 



ones undergo radical changes and assume the structure of 

 dicotyl stems. In fact, the section of old dicotyl roots looks so 

 much like that of a stem that it is often difficult to distinguish 

 it from a stem section. These changes can easily be followed 

 by making sections of a root at various distances behind the 

 growing point. 



Fig. 60. Cross-section of root of May Apple, showing radial bundle, a, endodermis ; 

 b, pericambiuin ; c, xyleru ray ; d, phi 

 ing the bundle (reduced, from Bastm), 



b, pericambiuin ; c, xyleru ray ; d, phloem ; p, parenchyma of cortex surround- 



tm). 



CHAPTER XXIII. 

 LEAVES. 



The leaf consists of: (1) the fibro-vascular system or frame- 

 work of veins; (2) the parenchyma or filling; (3) the epidermis, 

 which covers the whole leaf. The parenchyma or mesophyll of 

 the leaf is arranged differently in different leaves, giving rise 

 to two types of leaves, namely, Mfacial and centric. 



BIFACIAL LEAF. These are always flat leaves, and in section 

 present a distinct upper and lower surface, which are quite dif- 

 ferent in structure. The parenchyma cells next the upper sur- 

 face are compactly arranged and elongated perpendicular to the 

 surface. Such cells are known as palisade parenchyma. They 

 contain numerous chlorophyll bodies, which give to the upper 

 surface of such leaves the deeper green color, as compared with 

 the lower surface. 



The parenchyma next to the lower surface is loosely arranged 



