STRUCTURE OF M ONO C T Y L ED N U S STEM. 



377 



ness of its texture (Fig. 164). The bundles are least numerous 

 in the centre of the stem, and become gradually more approxi- 

 mated towards the circumference : but it frequently happens that 

 the portion of the area in which they are most compactly 



FIG. 163. 



FIG. 164. 



Transverse section of stem of young Palm. 



Portion of transverse section of stem 

 of Wunghie Cane. 



arranged, is not absolutely at its exterior, this portion being 

 itself surrounded by an investment composed of cellular tissue 

 only; and sometimes we find the central portion, also, com- 

 pletely destitute of fibro-vascular bundles ; so that a sort of 

 indication of the distinction between pith, wood, and bark is 

 here presented. This distinction, however, is very imperfect ; 

 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 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, also, 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 continuity with the leaves which are successively evolved at 

 its summit. It was formerly supposed that these successively 



