Inconvertibility of Bark into Alburnum . 107 
thing at all corresponding with the circular row of large tubes 
in the alburnum of the oak is discoverable in the bark of that 
tree. These tubes are also generated within the interior 
surface of the bark, which is well defined ; and during their 
formation the vessels of the bark are distinctly visible, as 
different organs ; and had the one been transmuted into the 
other, their progressive changes could not, I think, possibly 
have escaped my observation : nor does the organization of 
the bark in other instances, in any degree indicate the cha- 
racter of the wood that is generated beneath it : the bark of 
the wych elm ( ulmus montana ) is extremely tough and fibrous ; 
and it is often taken from branches of six or eight years old, 
to be used instead of cords ; that of the ash (fraxinus excel- 
sior ) on the contrary, when taken from branches of the same 
age, breaks almost as readily in any one direction as in an- 
other, and scarcely presents a fibrous texture ; yet the albur- 
num of these trees is not very dissimilar, and the one is often 
substituted for the other in the construction of agricultural 
instruments. 
Mirbel has endeavoured to account for the dissimilar orga- 
nization of the bark, and of the wood into which he conceives 
it to be converted, by supposing that the cellular substance of 
the bark is always springing from the alburnum, whilst the 
tree is growing, and that it carries with it part of the tubular 
substance ( tissu tubulaire ) of the liber, or interior bark. These 
parts of the interior bark, which are thus removed from con- 
tact with the alburnum, he conceives to constitute the external 
bark or cortex, whilst the interior part of the liber progres- 
sively changes into alburnum. 
But if this theory (which I believe I have accurately stated, 
P 2 
