ORGANIZATION OF THE STEM. 63 



the horizontal cellular system, condensed by pressure of the 

 longitudinal wedges into fine lines of cellular matter. 



On the whole, the organization of an exogenous stem, con- 

 sidered collectively, presents three distinct systems, the cortical, 

 ligneous and medullary system, or the bark, the wood and the 

 pith ; and from the mode of its development, it is evident that 

 the wood has a constant tendency to solidify itself, and the 

 bark to destroy itself; hence vitality soon ceases in the former, 

 and the exfoliation and fall of the different parts of the latter, 

 first the'epidermis, then the suberous cellules, the cortical pith 

 and even the liber. 



The stem of an old exogenous tree, therefore, consists of a 

 curious conjunction of dead and living matter, and the rings of 

 wood not only mark the growths of successive years, but the 

 number of generations of spontaneously grafted individuals 

 which the stem has sustained. No part of such a tree is alive 

 now that was living a few years ago. The leaves have fallen 

 which the tree then bore, and the nodes from which they 

 sprung are deeply buried in the interior of the stem, beneath 

 the wood formed by the generations of buds and leaves that 

 succeeded them; whilst the living bark that then covered the 

 stem in immediate contact with the wood has been separated 

 from it by the internal growth and deposition of other strata 

 of bark, and is now visible on the outside of the stem in the 

 form of dead and fissured layers, or else has been thrown oif 

 from its surface altogether. Thus in the coral tree, far beneath 

 the ocean wave, where mineral matter assumes a vegetable 

 form the recent shoots and surface alone are alive, all is dead 

 along the central axis. 



