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 
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 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 intermixture. There are, however, some 
difficulties 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 their bark, too. 
