GROWTH OF THE FIBRO-VASCULAR BUNDLES. 97 
of endogenous is properly understood, it can lead to no confusion 
in its application. 
As the fibro-vascular bundles of an endogenous stem, in the 
course of their successive development, are always directed at 
first towards the centre, it must necessarily follow that those 
previously 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 (jigs. 177, f, and 
192, A, b, c, d). 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 fibro- 
vascular bundles, the constituents of which become, moreover, 
more or less thickened in their interior, will be harder than the 
Fic. 196. Fig. 197. Fia. 198. 
Fig. 196. The Dragon Tree of Teneriffe (Dracena Draco), now destroyed. 
——Fig. 197. Dicotyledonous stem, with a woody twining plant aroundit. 
—Fig. 198. Monocotyledonous stem encircled by a woody twiner. 
upper. Therind in like manner, at the lower part, will become 
harder, from the greater number of liber-cells which terminate 
init. As endogenous stems increase in diameter, partly by the 
formation of fibro-vascular bundles in their interior, and partly 
by the general development of the parenchymatous tissue in 
which they are placed, it follows that as soon as the rind has 
become thus hardened by the liber-cells and other causes, it is 
not capable of further distension, and the stem will conse- 
quently become at length choked up by the bundles which con- 
tinue to descend, and further growth is then impossible. It 
is evident, therefore, that endogenous stems, unlike those of 
H 
