CHAP. III. 
ORIGIN OF WOOD. 
259 
hence a section of the wood of that plant has a pale, smooth, 
homogeneous appearance ; but in the wild cherry the medullary 
plates are much thicker, they adhere to the bark by deep 
broad spaces, and are arranged with great irregularity, so that 
a section of the wood of that variety has a deeper colour, and 
a twisted, knotty, very uneven appearance. 
In Quercus sessiliflora the medullary rays are thin, and so 
distant from each other that the plates of wood between them 
do not readily break laterally into each other, if a wedge is 
driven into the end of the trunk in the direction of its cleav- 
age : on the contrary, the medullary rays of Quercus pedun- 
culata are hard, and so close together that the wood may be 
rent longitudinally without difficulty ; hence the wood of the 
latter is the only kind that is fit for application to park- 
paling. 
As the medullary rays develope in a horizontal direction 
only, when two trees in which they are different are grafted 
or budded together, the wood of the stock will continue to 
preserve its own peculiarity of grain, notwithstanding its 
being formed by the woody matter sent down by the scion ; 
for it is the horizontal development that gives its character 
to the grain, and not the perpendicular fibres which are 
incased in it. 
The WOOD is at once the support of all the deciduous organs 
of respiration, digestion, and fertilisation, the deposit of the 
secretions peculiar to individual species, and also the reservoir 
from which newly forming parts derive their sustenance until 
they can establish a communication with the soil. 
Regarding the precise manner in which it is created, there 
has been great diversity of opinion. Linnaeus thought it was 
produced by the pith ; Grew, that the liber and wood were 
deposited at the same time in a single mass which afterwards 
divided in two, the one half adhering to the centre, the other 
to the circumference ; Malpighi conceived that the wood of 
one year was produced by an alteration of the liber of the 
previous season. Duhamel believed that it was deposited by 
the secretion already spoken of as existing between the bark 
and wood, and called cambium : he was of opinion that this 
cambium was formed in the bark and became converted into 
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