142 - Mr. Knight on the office 
believe, very generally supposed to ascend, appeared to have 
become impervious upon the conversion of alburnum into 
heart wood ; whilst the lateral or convergent cellular pro- 
cesses remained open to permit the ingress of the moving 
fluid, without which the heart wood could not probably long 
retain life. 
I must therefore reject the hypothesis which assumes the 
ascent of the sap through the heart wood, and must believe 
that the fluid, which affords the organizable matter that is 
annually deposited in the conversion of alburnum into heart 
wood, and which subsequently gives greater solidity and 
strength to that substance, is derived from the bark ; and 
that it passes inwards during the latter part of the summer 
and autumn, through the convergent cellular (or medullary) 
processes, to return in part through the same passages when 
a new layer of bark is to be formed in the spring. Under 
such circumstances the operation of the heart wood, when it 
exists in large quantity comparatively with the bark and 
foliage, as in sound pollard oak trees, must tend to check 
and diminish, rather than to promote, growth ; and amongst 
trees of this description I have often been able readily to 
select such as were sound by the slowness of the growth of 
their branches comparatively with those of other trees of 
apparently the same age, which were become hollow. 
Whether the heart wood of oak trees, which are deprived 
of their bark in the spring, and suffered to remain standing 
till the following winter, recover the whole, or a part of the 
good qualities which it loses ( or is supposed to lose) in the 
spring, is a very interesting question. A few experiments 
