INTERNAL STRUCTURE. 



73 



sooner or later ceases to distend or adapt itself to further in- 

 crease in diameter, and there is no interior provision for indefinite 

 increase in the greater number of woody endogenous trunks. But 

 in Dracaena (Dragon-trees) , in the arborescent Yuccas, and the 

 like, the zone intermediate between the cortical and interior re- 

 gion, which is for a tune active in many Endogens, here grows 

 continuously and indefinitely. Such trunks increase in diameter 

 throughout life ; they may attain a very great age (as some 

 Dragon-trees have done) ; and they imitate exogenous trunks 

 to a considerable extent in mode of growth. 



139. The wood of an endogenous woody stem is hardest and 

 most compact at the circumference ; in palm-stems commonly 

 it is largely mixed with parenchyma or pith at the centre, even 

 in old trunks. 



140. The Exogenous Structure, that of ordinary wood, is char- 

 acterized by the formation of a distinct zone of wood between a 

 central cellular medullary portion (pith) and an outer chiefly 

 cellular portion (bark) , traversed by plates from the pith (medul- 

 lary rays) , and b}- increasing from the outer surface of this zone 

 between wood and bark, the increase in enduring stems consist- 

 ing of definite concentric annual layers. 



141. Its Beginning, at the earliest growth of the embryo, is in 

 the appearance of a few ducts (Fig. 125,/-/), at definite points 

 in the common parenchyma of the initial stem (four equidistant 

 ones in the Sugar Maple) ; each is soon surrounded by incipient 



proper wood-cells (Fig. 125, 5, c), together forming a fibro-vascular 

 bundle or thread. Additional ones are intercalated as the second 

 and third internodes develop, and so a column (in cross- 

 section a ring) of wood is produced, always so arranged as to 



FIG. 128. Diagram of a cross-section of a forming seedling stem, showing the 

 manner in which the young wood is arranged in the cellular system. 



FIG. 129. The same at a later period, the woody bundles increased so as nearly to 

 fill the circle. 



FIG. 130. The same at the close of the season, where the wood lias formed a com- 

 plete circle, interrupted only by the medullary rays, which radiate from the pith to 

 the bark. 



