34 Disunion of contiguous Layers, Sfc. 



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 probably 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. 



3 



