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 fibro-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 ever 

 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 un frequently disappear after a 

 time, except at the nodes, leaving 

 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 



FIG. 539. Portion of transverse 

 section of stem of Wanghie cane. 



