98 FIBRO-VASCULAR BUNDLES IN DRACAINAS, ~ 
exogenous growth, cannot increase in diameter beyond a certain 
limit, and that from the same causes also they cannot live beyond 
a certain age. 
Although, as a general rule, the stems of Palms and most 
other Monocotyledons are thus limited in size and life, there 
are some remarkable exceptions to this, as for instance in 
Yuccas, and the Dracenas or Dragon-trees (fig. 196); in these 
the rind is always 
soft and capable of 
distension, and the 
fibro-vascular bun- 
dles, after having 
reached it, are con- 
tinued downwards as 
fibrous layers be- 
tween it and the 
original fibro - vascu- 
lar bundles, and thus 
form a sort of wood 
beneath, in success- 
ive layers, somewhat 
after the manner that 
layers of wood are 
produced by the cam- 
bium layer of an exo- 
genous stem. Such 
endogenous stems, 
like those of exoge- 
nous growth, have 
necessarily no limit 
either to their size 
or age. 
It is in conse- 
quence of the com- 
paratively small in- 
Fie. 199, 
Se een crease in diameter 
<A Sm Ne Ps A te bs 
aN ogee ca which most endoge- 
Fig. 199. 1. Unbranched stem of the Cocoa-nut Palm Nous stems undergo 
(Cocos nucifera), witha tuft of leavesatthesummit. after they have ar- 
2. Branched stem of Pandanus odoratissimus, with a i wed’ene bas & 
number of aerial roots arising from its lower part, rived a a cer aln age, 
and each branch terminated by a tuft of leaves. that twining plants 
The figures are placed at the base to give some “ 
idea of the height of the trees. which encircle them 
after that period has 
arrived, do them no injury, frequently not even producing the 
slightest swelling on their surface; thus proving incontestably 
that such stems do not increase in diameter after a certain age. 
The effect of such twining plants is well seen in fig. 198. If we 
compare this figure with that of an exogenous stem (jig. 197), 
with a woody twiner encircling it, we find a striking difference ; 
