82 ORGANOGRAPHY. BOOK I. 



wards; so that the centre, being always most newly formed, 

 is the softest ; and the outside, being older, and being gra- 

 dually 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 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. 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 show that 

 tins 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 course 

 of the stem for some distance, and then turns outward again, 

 finally losing itself in the cortical integument. In the course of 

 their downward descent, the woody bundles gradually separate 

 into threads, till at last the vascular system, which for a long 

 time formed an essential part of each of them, disappears, and 

 there is nothing left but woody tissue. In this view of the 

 growth of Endogens, the trunk of such plants must consist of 

 a series of arcs directed from above inwards, and then from 

 within outwards ; and consequently the woody fibres of such 

 plants, instead of being parallel with each other, must be inter- 

 laced in infinite intermixtin'e. There are, however, some 

 difiiculties in the way of this theory, which we do not find 

 adverted to by its author. If Mohl's view of the structure of 

 Endogens be correct, they must after a time lose the power of 

 growing, in consequence of the whole of the lower part of their 

 stems being choked up by the multitude of descending woody 

 bundles. Is this the case ? The lower part of dieir bark, too, 



