wood of Exogenous Trees. 33 



but that it may be explained in a simple manner independently of 

 that theory. 



I possess a specimen strikingly similar to that described by Dr 

 Lindley. It consists of about two feet of the trunk of a poplar six 

 inches in diameter, which has been split longitudinally. In the 

 centre is exposed the earlier state of this trunk, when it was about 

 two inches in diameter, and the want of union between this central 

 portion and the surrounding layers is complete. Upon the inner ■ 

 trunk (as we may call it) are the stumps of some pruned branches 

 quite as distinct as in Dr Lindley's specimen. The wood of this 

 inner trunk is sound except for some distance along its surface, on 

 one side only. At the lower extremity of the specimen the layers 

 of the outer trunk are regularly formed, being concentric with those 

 of the inner, and possessing every appearance of having been depo- 

 sited in the usual manner ; and a transverse section would not have 

 exhibited any want of organic union between the two trunks. But 

 at the upper extremity, the layers of the outer trunk are not so re- 

 gular. Those which lie next the inner trunk, instead of being con- 

 tinuous, are interrupted and curved in- fig. a. 

 wards (fig. a.) exactly as we see them 

 in all cases where a wounded surface has 

 become coated over with fresh wood. A 

 patch of bark is enclosed between these 

 layers and the inner trunk, and it is on 

 this side that the surface of the latter is 

 partially decayed. In short, the appear- 

 ance is precisely that which would be presented if one side of the 

 trunk had died whilst the other continued to grow, and spread gra- 

 dually over the dead portion ; and I cannot but conclude that such 

 has really been the case, notwithstanding this want of organic con- 

 nection between the inner and outer trunk. This want of union, 

 therefore, does not arise from the death of the inner trunk pre- 

 viously to the deposition of the layers which are formed round it, as 

 Dr Lindley supposes to have been the case in his specimen, but 

 must have resulted from the action of some cause subsequent to the 

 formation of the layer in immediate contact with the inner trunk, 

 which layer may therefore have been deposited in the usual man- 

 ner ; without the necessity of our having recourse to the explana- 

 tion afforded by the theory of Du Petit Thouars. In fact, accord- 

 ing to Dr Lindley himself, this theory only explains the formation 

 of the vascular portion of the layers, it being allowed that the cel- 

 lular portion arises from a lateral development of this tissue in the 



