EXOGENOUS STRUCTURE. 



117 



soon appear, and form the earliest ■woody tissue. As the rudiments 

 of the next internode and its leaves appear, two or four additional 

 threads of vascular tissue appear in the stem below, in the paren- 

 ch}'ma between the earliest ones, and are equally surrounded with 

 forming Avoody tissue. At an early stage, therefore, the developing 

 stem is seen to be traversed by several bundles of woody tissue, with 

 some vessels imbedded ; and these, as they increase and enlarge, run 

 together so as to make up a woody zone (or, as seen in the cross- 

 section, a ring), enclosing the central part of the parenchyma within 

 it, and itself enclosed by the extei-nal parenchyma. Thus a zone or 

 layer of wood is formed, which is so situated in the original homo- 

 geneous cellular system as to divide it into two parts ; namely, a 

 central portion, which forms the pith, and an exterior portion, Avhich 

 belongs to the bark. The whole is of course invested by the skin 

 or epidermis, which covers the entire surface of the plant. The 

 way in which the layer of wood thus originates is somewhat rudely 

 illustrated by the aimexed diagrams (Fig. 186- 188). The several 



woody masses, or wedges, are separated from each other by lines or 

 bands of the original cellulm- tissue, Avhich pass from the pith to the 

 bark, and which necessarily become narrower and more numerous as 

 the woody bundles or wedges increase in size and number. These 

 bands are the 



209. Medullary Rays. These form the radiating lines that the 

 cross-section of most exogenous wood so plainly exhibits, especially 

 that of the Oak, Plane, &c. They consist of parenchyma, more 



FIG. 186. Plan of a cross-section of a forming seedling stem, showing the manner in which 

 the young wood is imbedded in the cellular system. 



FIG. 187. The same at a later ijeriod, the woody bundles increased so as nearly to fill the 

 circle. 



FIG. 188. The same at the close of the season, where the wood has formed a complete 

 circle, separating the pith from the bark, except that they are still connected by narrow por- 

 tions of the cellular system (the medullary rays) which radiate from the pith to the bark. 



