CHAP. V. 
ORIGIN OF WOOD. 
315 
further evidence were required, it would be furnished by a 
case adduced by Achille Richard, who states that he saw, in 
the possession of Du Petit Thouars, a branch of Robinia 
Pseudacacia on which R. hispida had been grafted. The 
stock had died; but the scion had continued to grow, and 
had emitted from its base a sort of plaster formed of very dis- 
tinct fibres, which surrounded the extremity of the stock to 
some distance, forming a kind of sheath; and thus demon- 
strating incontestably that wood does descend from the base 
of the scion to overlay the stock. The singular mode of 
growth in Pandanus is equally instructive. In that plant, the 
stem, next the ground, is extremely slender ; a little higher 
up it is thicker, and emits aerial roots, which seek the soil and 
act as stays upon the centre. As the stem increases in height, 
it also increases notably in diameter, continuing to throw out 
aerial roots. If the roots were pruned away, the stem would 
be an inverted cone ; but, if we add to the actual thickness of 
the base of the stem the capacity of the aerial roots at that 
part, the two together will be about equal to the capacity of 
the stem at the apex ; which suggests the idea that the woody 
matter that descends from the leaves may really be their 
roots, passing through the horizontal cellular system of the 
stem. An analogous but much more remarkable case is the 
following mentioned by me in the Penny Cyclopcedia^ article 
Endogens, vol. ix. p. 396. In an unpublished species of Bar- 
bacenia from Rio Janeiro, allied to B. purpurea, the stems 
appear externally like those of any other rough-barked plant, 
only that their surface is unusually fibrous and ragged when 
old, and closely coated by the remains of sheathing leaves 
when young. Upon examining a transverse section of it, 
the stem is found to consist of a small, firm, pale, central circle 
having the ordinary endogenous organisation, and of a large 
number of smaller and very irregular oval spaces pressed 
closely together, but having no organic connection ; between 
these are traces of a chaffy ragged kind of tissue which 
seems as if principally absorbed and destroyed. (See 
Jig, 192.) 
A vertical section of the thickest part of this stem exhibits, 
in addition to a pale, central, endogenous column, woody 
