ORGANS OF KTTRITION. 87 



as they do ultimately at the circumference. On this account the 

 name endogenous has been altogether discarded of late years by 

 many botanists, who use instead that of riumocotyledonous, a 

 term, as already noticed, derived from the embrj-o of plants with, 

 such stems possessing but one cotyledon. In this volume we have 

 employed both terms, but more frequently that of endogenous, 

 because this is the one by which such stems have been known 

 for a long period, and is that therefore which is best understood. 

 As the vascular bundles of an endogenous stem, in the course 

 of their successive development, are always deposited at first 

 towards the centre, it must necessarily follow that those pre- 

 viously formed will be gradually pushed outwards, for which 

 reason the outer part of a transverse section will always exhibit 

 a closer aggregation of bundles than the inside {jigs. 156 and 

 171, a). In such stems, therefore, the hardest part is on the out- 

 side, and the softest inside, directly the reverse of what occurs in 

 those of exogenous growth. The lower portion of such stems 

 also, in consequence of the descent of the vascular bundles, which 

 become, moreover, more incrusted by secondary deposits, will be 

 harder than the upper. The rind in like manner, at the lower 

 part, will become harder, from the greater number of liber-cells 

 which terminate in it. As endogenous stems increase in diameter, 

 partly by the deposit of vascular bundles in their interior, and 

 partly by the general development of the parench^nnatous tissue 

 in which they are placed, it follows that as soon as the rind or 

 false bark has become thus hardened by the liber-cells, it is not 

 capable of further distension ; and the stem will consequently 

 become at length choked up by the bundles which continue to 

 descend, and further growth is then impossible. It is evident, 

 therefore, that endogenous stems, unlike those of exogenous 

 growth, cannot increase in diameter beyond a certain limit, and 

 that from the same causes also they cannot live beyond a certain 

 age. 



Although, as a general rule, the stems of Palms and most 

 other monocotyledonous plants are thus limited in size and life, 

 there are some remarkable exceptions to this, as for instance in 

 Yuccas, and the BraccBnas or Dra^/on-trees {fig. 11%) ; in these the 

 rind is always soft and capable of distension, and the vascular 

 bundles, after having reached it, are continued downwards as 

 fibrous layers between it and the original vascular bundles, and 

 thus form a sort of wood beneath, in successive layers, somewhat 

 after the manner that layers of wood are produced by the cam- 

 bium-layer of an exogenous stem. Such endogenous stems, like 

 those of exogenous growth, have necessarily no limit either to 

 their size or age. 



It is in consequence of the comparatively small increase in 

 diameter which most endogenous stems undergo after they have 

 arrived at a certain age that twining plants which encircle them 



