CHAP. V. 
ORIGIN OF WOOD. 
317 
at the base of the stem, than near the upper extremity. No- 
thing can well show more distinctly than this, that the woody 
bundles of the endogenous stem are a sort of roots emitted by 
the leaves, plunging down through their whole length into 
the cellular substance of the stem in ordinary cases ; but, in 
Barbacenia, soon quitting the stem, and continuing their 
course downwards on the outside. The observation of Du 
Petit Thouars, that, when Dracaenas push forth branches, 
each of the latter produces from its base a quantity of fibres, 
which are interposed between the cortical integument and 
the body of the wood, forming a sort of plaster analogous to 
what is found in the graft of an Exogen ; and that, of the fibres 
just mentioned, the lowermost have a tendency to descend, 
while those originating on the upper side of the branch 
turn downwards, and finally descend also ; had already 
rendered the above-mentioned conclusion probable. The 
case of Barbacenia can scarcely leave a doubt upon the 
subject; and leads to the important conclusion, that the theory 
of the wood of Exogens being also a state of roots belonging 
to the leaves of the stem is well founded also. 
Mirbel, who formerly advocated the doctrine of wood 
being deposited by bark, has candidly admitted the opinion 
to be no longer tenable ; and he has suggested, in its room, 
that wood and bark are independent formations, which is 
no doubt true ; but he adds, created out of cambium, in 
which it is impossible to concur, if by cambium M. Mirbel 
means the viscid secretion found in the spring between the 
bark and wood of Exogens ; for the following reasons : — All 
the writers hitherto mentioned have considered the formation 
of wood exclusively with reference to exogenous trees, and to 
such only of them as are the common forest plants of Europe. 
Had they taken into account exotic trees or any endogenous 
plants, they would have seen that none of their theories 
could apply to the formation of wood in the latter tribe. 
In many Exogens of tropical countries, wood is not de- 
posited in regular circles all round the axis, but only on 
one side of the stem, or along certain lines upon it : were it 
a deposit from the bark, or a metamorphosis of cambium, it 
would necessarily be deposited with some kind of uniformity. 
