STRUCTURE OF MONOCOTYLEDONOUS STEM. 



407 



Fig. 213. 



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 section by the closeness of its texture 

 (Fig. 213). The bundles are least numerous in the centre of the 

 Stem, and become gradually more approximated towards its cir- 

 cumference : but it frequently happens that the portion of the area 

 in which they are most com- 

 pactly arranged is not abso- 

 lutely at its exterior, this 

 portion being itself sur- 

 rounded by an investment 

 composed of Cellular tissue 

 only ; {and sometimes we find 

 the central portion, also, 

 completely destitute of Fibro- 

 Vascular bundles ; so that a 

 sort of indication of the dis- 

 tinction between Pith, Wood, 

 and bark is here presented. 

 This distinction, however, 

 is very imperfect ; for we 

 do not find either the cen- 

 tral or the peripheral por- 

 tions ever separable, like 

 Pith and Bark, from the in- 

 termediate Woody layer. In 

 its young state the centre of 

 the Stem is always filled-up 

 with Cells ; but these not 

 unfrequently 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 towards the surface of the Stem, and assist, 

 by their interlacement with the outer bundles, in forming that 

 extremely tough investment which the lower ends of these stems 

 present. The Fibro-Vascular bundles once formed receive no 

 further additions ; and the augmentation of the Stem in diameter 

 depends upon the development of new Woody bundles, in con- 

 tinuity with the leaves which are successively evolved at its 

 summit. It was formerly supposed that these successively-formed 

 bundles descend in the interior of the Stem through its entire 

 length until they reach the Roots ; and as the successive develop- 

 ment of Leaves involves a successive development of new bundles, 

 the Stem was imagined to be continually receiving additions to its 



Portion of Transverse Section of 

 Stem of Wanghie Cane. 



