ORGANS OF NUTRITION. 93 



cotyledonous. In this volume we generally use the term endo- 

 genous, because it is the one by which such stems have been 

 known for a long period, and is accordingly that 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 {jiys. 164 and 179, 

 A). In such stems, therefore, the hardest part is on the outside, 

 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 be- 

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

 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 dia- 

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

 and partly by the general development of the cellular 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; the stem, therefore, becomes at 

 length choked up by the descending bundles, and growth is no 

 longer possible. It is evident, therefore, that endogenous stems, 

 unlike those of exogenous, cannot grow in diameter beyond a cer- 

 tain 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 Endogens are thus limited in size and life, 

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

 Yuccas, the Draccenas or Dragon-trees {fig. 186), &c. ; in these 

 the rind is always soft and capable of distension, and the vascu- 

 lar 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, somcAvhat 

 after the manner that layers of wood are produced by the 

 cambium-layers of Exogens. Such endogenous stems, there- 

 fore, like those of exogenous, have no limit either to their size or 

 age. 



It is in consequence of the comparatively small increase in 

 diameter which most endogenous stems undergo, that twining 

 plants which encircle them do them no injury, frequently not 

 even producing the slightest swelling on their surface, and thus 

 proving incontestably that such stems do not increase in dia- 

 meter after a certain age. The effect of such climbers is well 

 seen in fig. 184. If we compare this figure with that of an 

 exogenous stem {fig. 183), Avith a woody twining plant encir- 

 cling it, we find a striking difference ; for here we observe exten- 



