STRUCTURE OF STEMS 



701 



ducts, which are at once distinguished by the size of their openings ; 

 and these are surrounded by woody fibre and spiral vessels, the 

 transverse diameter of which is so extremely small that the portion 

 of the bundles which they form is at once distinguished in transverse 



FIG. 538. Transverse section of stem of young palm. 



section by the closeness of its texture (fig. 539). The bundles are 



least numerous in the centre of the stem, and become gradually more 



crowded towards its circumference ; but it frequently happens that the 



portion of the area in which they are 



most compactly arranged is not abso- 



lutely at its exterior, this portion being 



itself surrounded by an investment 



composed of cellular tissue only ; and 



sometimes we find the central portion 



also completely destitute of tibro- vas- 



cular bundles ; so that a sort of indica- 



tion of the distinction between pith. 



wood, and bark is here presented. 



This distinction, however, is very im- 



perfect ; for we do not find either the 



central or the peripheral portions e\ in- 



separable, like pith and bark, from 



the intermediate woody layer. In its 



young state the centre of the stem is 



always filled up with cells ; but these 



not unfrequently disappear after a 



FIG. 589. Portion of 



time, except at the nodes, leaving section of stem of Wanghie 

 the stem hollow, as we see in the 

 whole tribe of grasses. When a vertical section is made of a woody 

 stem (as that of a palm) of sufficient length to trace the whole extent 

 of the fibro-vascular bundles, it is found that, whilst they pass at 

 their upper extremity into the leaves, they pass at the lower end 



