STEM OF EXOGENS. 



93 



and Bark. The pith is a soft spongy substance occupying the 

 centre ; if a thin slice of it be cut, either across or vertically, and 

 magnified, it is seen to consist entirely of cellular tissue, the cells 



Fro. 40. REKCH TREK ; EXOGEN. 



of which are mostly of a very regular form (. 71). "When 

 young it contains a good deal of fluid, and has a greenish hue ; 

 when the branch is older, it becomes white and dry ; and in an 

 old stem or branch, it is often found to have shrivelled up and 

 almost entirely disappeared. 



127. Now this pith is the first-formed portion of the Stem ; 

 being in fact the remainder of that cellular structure, of which 

 the whole was originally composed, but which gradually gives 



