54 



COMPOUND ORGANS OF PLANTS. 



wood, and pith which the stem exhibits on its transverse 

 section. 



Fig. u. 



<:/9- Jf 



Fig. 11 shows a transverse gection A, and vertical section B, of an exogenous or 

 dicotyledonous stem of three years* growth. *' In both sections, a represents the 

 cellular tissue of the pith, & h, the dotted ducts, and c Cj the woody fibre of the 

 successive annual layers; d d, the spiral vessels of the medullary sheath ; e e 

 cambium layer ; f f, liber ; g g, cellular envelope ; h h, corky layer ; i i, mediillary 

 rays. In the vertical section the medullary ray is shown in only part of its 

 length ; since the continuity of the medullary rays from the pith to the bark, owing 

 to the slight flexure which always ocours in them, is rarely or never shown by such 

 a section."* 



The Bark. — From this section, it is evident that the bark, 

 anatomically considered, may be subdivided into four parts. 



1. The epidermis or general outer integument. The nature 

 of this investment has been already examined pp. (18-24). It 

 only remains at present to add, that in forest trees and larger 



* See Carpenter's " Principles of Physiology, General and Comparative,' ' 

 3d edition, 1851. 



