BARK. 885 



the interior of all those which preceded it ; they also extend the 

 medullary rays, which still maintain a continuous connection be- 

 tween the pith and the bark; but a portion remains unconverted, 

 so as always to keep apart the liber and alburnum. This type 

 of stem structure is termed Exogenous ; a designation which ap- 

 plies very correctly to the mode of increase of the woody layers, 

 although (as we have just seen) the liber is formed upon a truly 

 endogenous plan. 



241. Numerous departures from the normal type are found 

 in particular tribes of Exogens. Thus in some, the wood is not 

 marked by concentric circles, their growth not being inter- 

 rupted by any seasonal change. In other cases, again, each 

 woody zone is separated from the next, by the interposition of 

 a thick layer of cellular substance. Sometimes wood is formed 

 in the bark (as in Calycanthus), so that several woody columns are 

 produced, which are quite independent of the principal woody 

 axis, but cluster around it. Occasionally the woody stem is 

 divided into distinct segments, by the peculiar thickness of cer- 

 tain of the medullary rays ; and in the stem of which Fig. 178 

 represents a transverse section, these cellular plates form four 

 large segments, disposed in the manner of a Maltese cross, and 

 alternating with the four woody segments, which they equal in 

 size. 



242. In its first- developed state, the Exogenous stem con- 

 sists, like the so-called endogenous, of cellular tissue only ; but 



FIG. 179. 

 FIG. 178. 



Fig. 178. Transverse section of the stem of a Climbing-plant (Aristolochia?) from New Zealand. 

 Fig.179. Portion of transverse section of Burdock (Arctium), showing one of the fibro-vascular 

 bundles, that lies beneath the cellular integument. 



after the leaves have been actively performing their functions for 

 a short time, we find a circle of fibro-vascular bundles, as repre- 

 sented in the diagram, p. 378, interposed between the central 

 (or medullary) and the peripheral (or cortical) portions of the 

 cellular matrix ; these fibro-vascular bundles being themselves 



25 



