34 Disunion of contiguous Layers, *c. 
older layers. It may readily be conceived that the excessive prun- 
ing, to which it is evident that Dr Lindley's poplar and my own 
specimen were subjected, might prevent the wood of that year's 
growth from becoming fully matured, and thus the connection be- 
tween it and the next layer might be so slight as to admit of their 
being readily detached after the latter had become fully indurated. 
This separation would be further assisted by the numerous surfaces 
of the pruned stumps, since no union whatever takes place between 
the older wood thus exposed and the new wood which forms over it, 
the very marks of the knife, which may happen to be on it when 
imbedded, will be accurately preserved. But FIG. B. 
independently of the cause, whatever it may be, 
which has operated in producing the separa- 
tion between the inner and outer layers of these 
specimens, it is by no means uncommon to find 
examples among the succulent branches of such 
wood as the willow, possessing a loose texture, 
where a slight degree of violence is sufficient 
to separate two contiguous layers, and I have 
found similar instances in large and dry branch- 
es of the elm. I possess a very good example 
from this latter tree, produced by the violence 
of the concussion with which a branch struck 
the ground when the tree was felled. The 
shock has detached some of the outer layers to- 
wards the extremity of the broken branch, and has thus left the 
inner layers exposed for some length, and exhibiting the perfect 
form which the branch possessed when it was much younger. (See 
FIG. B.) 
In the specimen of poplar which I have described, the inner trunk 
has not become so much contracted in drying as the surrounding 
layers, probably from its having been longer dead and more tho- 
roughly seasoned ; and the consequence is, that its extremities pro- 
ject beyond the truncated surfaces of the others. This fact points 
to another cause which may probahly assist in disuniting two con- 
tiguous layers, since one portion of the trunk may die and either 
contract or expand more than the other. 
The phenomenon we have been examining may be considered 
analogous to the growth of new wood round a decayed or hollow in- 
terior, or perhaps still more closely to the partial destruction of a 
newly formed layer by frost. A transverse section exhibits such a 
dead layer blackened and decayed, lying between two layers of 
sound wood. 
