Internal Modifications. 271 



f"r the very obvious arrangement of the veins in the leaves re- 

 veals the secret structure of the stem. 



plants of an arborescent habit having the endogenous struc- 

 ture are mostly natives of the tropics. In them the vascular and 

 cellular systems are as distinct as in the exogenous, but differ- 

 ently arranged. The cellular system, instead of being distin- 

 guishable into pith, bark, and medullary rays, is a uniform mass, 

 hi which the vascular system lies embedded in the form of thick 

 fibres ; and has itself no tendency to collect into zones or wedges 

 resembling wood, but in all cases retains the form of bundles 

 resembling fibres. These bundles consist of woody fibre enclos- 

 ing spiral vessels or ducts; most commonly the latter. The 

 diameter of an endogenous (literally inside growing) stem is 

 increased by the constant addition of fibrous bundles to the 

 centre, whence the name. These bundles displace such as are 

 previously formed, pushing them outwards; so that the centre 

 bein°" always the most newly formed, is the softest ; and the out- 

 side being older, and being gradually rendered more and more 

 compact by the pressure exercised upon the bundles lying next 

 it by those forming in the centre, is always the hardest. In en- 

 dogenous plants that attain a considerable age, such as many 

 palms, this operation goes on till the outside becomes sqmetimes 

 hard enough to resist the blow of a hatchet. It does not, how- 

 ever, appear that each successive bundle of fibres passes exactly 

 down the centre, or that there is even much regularity in the 

 manner in which they are arranged in that part; it is only 

 certain that it is about the centre that they descend, and that on 

 the outside no new formation takes place. Its epidermis seems 

 capable of very little extension, and in most plants of this kind 

 the diameter of the stem is the same, or not very widely differ- 

 ent, at the period in which it was first formed and when it has 

 arrived at its greatest age ; palms are in particular an instance 

 of this ; whence the cylindrical form is so common among 



them. 



As endogenous stems contain no concentric zones, there is 

 nothing in their internal structure to indicate age. In the grasses 

 the stem is hollow, except at the nodi, where transverse parti- 

 tions intercept the cavity, dividing it into many cells. In the 

 bamboo these cells and partitions are so large that, as is well 

 known, lengths of that plant are used as cases to contain pa- 



