184 EXOGENS AND ENDOGENS. [BOOK i. 



3. Of its Internal Modifications. 



In its internal structure the stem of Flowering plants is 

 subject to two principal and to several subordinate modifica- 

 tions. The former are well illustrated by such plants as the 

 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 integu- 

 ment or bark, having often a fibrous lining or liber, an inter- 

 mediate woody mass, and certain fine lines radiating from the 

 pith to the bark, through the wood, and called medullary 

 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 larger number of holes irregularly arranged, and caused by 

 the section of bothrenchymatous and vascular tissue, and of 

 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. The former constitutes the general mass of 

 the stem, the latter is found exclusively in a longitudinal 

 direction, and seems as if plunged amidst the cellular, like 

 roots into earth. The cellular or horizontal system may be 

 compared to the cross threads or woof in linen, the vascu- 

 lar or longitudinal system to the warp. It is only by a 

 diversity in their respective arrangement that the differences 

 above described are caused. In explaining in detail the pecu- 

 liar structure of Exogenous and Endogenous stems, I shall 

 continue to consider them with reference to those two sys- 

 tems, because that seems best adapted to the purposes of a 

 teacher. Hereafter some general views upon the subject will 

 be introduced to the reader's notice. 



