130 



THE STEM. 



by medullary rays. But it consists of bundles of woody and vas- 

 cular tissue, in the form of thick fibres or threads, which are im- 

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

 whole is inclosed 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 through- 

 out the cellular system of the stem, most copiously near the cir- 

 cumference, but without being arranged in layers. Each bundle 

 usually contains all the elements of the wood of the exogenous 

 stem, namely, vessels, proper woody tissue, and bast-cells. The 

 bundles 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 integu- 

 ment, or rind. As the stem increases, new bundles, springing 

 from the bases of more recently developed leaves, are at first di- 

 rected towards the centre of the stem, along which they descend 

 for a considerable distance, then, curving outwards, they mostly 

 terminate in the rind. It is partly in consequence of the cohesion 

 of these obliquely descending fibres to the false bark, that the lat- 

 ter cannot, as in Exogens, be separated from the wood beneath. 

 The manner in which the woody threads are consequently interwo- 

 ven is shown in Fig. 167. The palm- 

 like Yuccas of the Southern States offer 

 beautiful illustrations of the kind. The 

 appearance on a cross-section of an en- 

 dogenous stem is shown in Fig. 150. 

 The new woody bundles which are add- 

 ed from year to year, instead of ar- 

 ranging themselve outside the earlier 

 wood and inclosing it, as in Exogens, 

 actually descend more in the centre, and 

 gradually force outward those which 

 were first formed. Such a stem, there- 

 fore, instead of having the oldest and 

 hardest wood at the centre and the new- 

 est and softest at the circumference, as in ordinary trees, is softest 



FIG. 167. Vertical and transverse section of a young endogenous stem, to show the curv- 

 ing of the fibres. 



