324 COMPOSITE ORGANS. CHAP. II. 



pith seems to be besides essentially different from 

 that of the membrane composing the cells of the 

 pulp ; the former being evidently of a much 

 more elastic nature than the latter, as well as 

 much more capable of resisting the action of water, 

 when emptied of the parenchyma, as may be seen 

 by comparing them in their empty state. The 

 juiceless pith of the Elder or Bulrush is a good ex- 

 ample of the one ; and the withered pulp, or cellular 

 integument of the Lime-tree, is a good example of 

 the other. 



But although the pith has been described as in- 

 sulated and unmixed with any other organ, it is not 

 always entirely so. For it is occasionally to be 

 found interspersed with longitudinal fibres passing 

 throughout its whole extent, as may be seen in the 

 older branches of the Elder, or even in the annual 

 shoot in the course of the winter. In this case 

 they are easily distinguished by their colour, which 

 is a sort of rusty brown, while that of the pith is 

 white, in the surface of which they lay imbedded. 

 Perhaps this is the commencement of the process 

 by which the pith is finally obliterated or con- 

 verted into wood ; and the origin of the circular 

 row of longitudinal fibres, known by the appellation 

 of the Medullary Sheath. 

 4 



