262 
PHYSIOLOGY. 
BOOK II. 
had died; but the scion had continued to grow, and had 
emitted from its base a sort of plaster formed of very distinct 
fibres, which surrounded the extremity of the stock to some 
distance, forming a kind of sheath ; and thus demonstrating 
incontestably that fibres do descend from the base of the scion 
to overlay the stock. The singular mode of growth in Pan- 
danus 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. As 
it really grows, the stem, if the roots were pruned away, 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; showing that, unless the roots descend 
among the wood, the stem will not increase in diameter. 
Mirbel, who formerly advocated the doctrine of wood being 
deposited by bark, has, with the candour of a man of real 
science, fairly admitted the opinion to be no longer tenable ; 
and he has suggested in its room that wood and bark are in- 
dependent formations, — which is no doubt true, — but, he 
adds, created out of cambium, in which it is impossible to 
concur ; for this reason. All the writers hitherto mentioned 
or adverted to have considered the formation of wood only 
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 possibly apply to 
the formation of v>^ood in that tribe. In many exogenous 
plants of tropical countries, wood is not deposited 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. In endogenous trees 
there is no cambium, and yet wood is formed in abundance ; 
and the new wood is created in the centre, and not in the 
circumference : so that bark can have, in such cases, nothing 
whatever to do with the creation of w^ood. 
