CHAP. II. 



EXOGENOUS STEMS. 



67 



Oak and the Cane, specimens of which can be easily obtained 

 for comparison. A transverse slice of the former exhibits a 

 central cellular substance or pith, an external cellular and 

 fibrous ring or hark, an intermediate woody mass, and certain 

 fine lines radiating from the pith to the bark, through the 

 wood, and called viedullary rays ; this is called Exogenous 

 structure. In the Cane, on the contrary, neither bark, nor 

 pith, nor wood, nor medullary rays, are distinguishable ; but 

 the transverse section exhibits a large number of holes irregu- 

 larly arranged, and caused by the section of vasiform tissue, 

 and the mass of woody and cellular substance in which they 

 lie imbedded. This kind of structure is named Endogenous. 

 In both cases there is a cellular and vascular system distinct 

 from each other ; it is only by a diversity in their respective 

 arrangement that the differences above described are caused. 

 In explaining in detail the peculiar structure of Exogenous 

 and Endogenous stems, it will be more convenient to consider 

 them with reference to those two systems, than to follow the 

 usual method of leaving the fact of there being two distinct 

 systems out of consideration. 



§ 1. Of the Exogenous Structure. 



31 



The cellular system in an Exogenous stem chiefly occupies 



F -2 



