72 
COURSE OF THE SAF. 
[Part II. 
Received theo¬ 
ries of growth 
in girthing. 
wood of very different qualities is deposited on 
the stem of the same grafted tree. 
Of the fact which I have supposed probable, 
namely, the lateral communication of the sap 
throughout the wood to the bark, I will give 
proof immediately, while considering the office 
of the pith (or medulla, “marrow”), and its 
rays or silver grain (medullary rays), which 
many have supposed to be the means of this 
lateral communication between the pith and the 
bark. But I will first state those theories, in 
reference to the growth in girthing, which have 
been most generally accepted. 
Grew said that the new layer of wood is 
formed from a viscid substance, to which he 
gave the name of cambium . But where is the 
cambium itself formed ? How and where elabo¬ 
rated? Duhamel thought that the last year's 
layer of bark was converted into this year’s 
layer of wood. We can see that this is not the 
case. Du Petit-Thouars thinks that the new 
layer of wood is formed by the buds (though I 
believe the origin of this theory is due to Dar¬ 
win) ; that the fibres in the new layer of wood 
are the roots of the buds, which, at the bursting 
of the buds, run between the last year’s bark 
and wood to the ends of the roots. 
