ENDOGENOUS STRUCTURE. 



115 



204. The stem of an Endogen, as already explained (199), offers 

 no manifest distinction into bark, pitli, and wood ; and the latter is not 

 composed of concentric rings or layers. But it consists of bundles of 

 woody and vascular tissue, in the form of fibres or threads, which are 

 imbedded, with little apparent regularity, in cellular tissue ; and the 

 whole is enclosed in an integument, which does not strictly resemble 

 the bark of an Exogenous plant, inasmuch as it does not increase 

 by layers, and is never separable from the wood. The fibrous 

 bundles which compose the wood, and which consist of a mass of 

 woody fibres surrounding several vessels, are distributed throughout 

 the cellular system of the stem, but most abundantly towards the 

 circumference. Each bundle usually contains all the elements of 

 the wood of the exogenous stem ; namely, vessels, proper woody 

 tissue, and bast-cells. The bundles often may be traced directly 

 from the base of the leaves down through the stem, some of them 

 to the roots in a young plant, while others, curving outwards, lose 

 themselves in the cortical integument, 



or rind. As the stem increases, new 

 bundles, springing from the bases of 

 more recently developed leaves, are at 

 first directed towards the centre of the 

 stem, along which they descend for 

 some distance, growing more slender 

 in their course, and then, curving out- 

 Avards, mostly terminate in the rind. 

 It is partly in consequence of the co- 

 hesion of these obliquely descending 

 fibres to the false bark, that the latter 

 cannot, as in Exogens, be separated 

 from the wood beneath. The manner 

 in which the woody threads are consequently interwoven is shown 

 in Fig. 185. The Palm-like Yuccas of the Southern States offer 

 beautiful illustrations of the kind. 



205. Endogenous stems, instead of having the oldest and hardest 

 wood at tlie centre and the newest and softest at the circumference, 

 as in ordinary trees, are softest towards the centre and most compact 

 at the circumference. They increase in diameter with the increas- 

 ing number of woody bundles (which multiply as new leaves are 



185 



FIG. 185. Vertical and transverse section of a young endogenous stem, showing the curv- 

 ing of the fibres. 



