222 STRUCTURE OF ENDOGENS. [BOOK i. 



the investigations of Mohl and others have thrown great light 

 upon their real organisation. 



In Endogenous plants the vascular and cellular systems 

 are as distinct as in Exogenous, but they are differently 

 arranged. The cellular system, instead of being distinguish- 

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

 in which the vascular system lies imbedded in the form of 

 thick fibres, seldom having any tendency to collect into zones 

 or wedges resembling wood. The fibrous bundles consist of 

 woody tissue, enclosing spiral or other vessels. 



The following is an explanation of the opinions generally 

 entertained concerning the formation of an Endogenous 

 stem. Its diameter is supposed to be increased by the con- 

 stant addition of woody bundles to the centre, whence the 

 name ; those bundles displace such as are previously formed, 

 pushing them outwards; so that the centre, being always 

 most newly formed, is the softest; and the outside, 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 the hardest. In Endogenous 

 plants that attain a considerable age, such as many Palms, 

 this operation goes on till the outside becomes sometimes 

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

 however, appear that each successive bundle of fibres passes 

 exactly down the centre, or that there is even much regu- 

 larity in the manner in which they are arranged in that part ; 

 it -is only certain that it is about their centre that they 

 descend, and that on the outside, below the growing point, 

 no new formation takes place from the circumference. This 

 appears from the manner in which the bundles cross and 

 interlace one another, as is shown in the figure of Pandanus 

 odoratissimus given by De Candolle in his Organographie 

 (tab. vi.), or still more clearly in the lax tissue of the inside 

 of the stems of Dracaena Draco. 



The investigations of Mohl appear, however, to prove that 

 this view of the structure of Endogens requires some modifi- 

 cation. According to this observer, every one of the woody 

 bundles of a Palm stem originates in the leaves, and is at 

 first directed towards the centre ; arrived there, it follows the 



