480 Mr. Lindley on an 



their fibrous and vascular tissue is held together by a cellular 

 substance which lies among them, assuming the form of plates 

 radiating from the centre to the circumference, and called 

 medullary rays. But it is apparent, from what has been 

 already stated, that if the origin of the wood and bark is such 

 as du Petit Thouars and his followers suppose, such an organic 

 connexion between the outside and inside of a trunk is not in- 

 dispensable to the formation of wood. 



Let us now consider which of the three last theories best 

 explains the structure of the specimen which is the subject of 

 this communication. The central pruned axis was no doubt 

 the original stem of the poplar, and was formed anterior to any 

 of the superj acent wood ; and the question is, how the latter 

 originated, without some organic connexion with the centre. 

 If we could suppose, with Grew or Malpighi, that bark pro- 

 duces wood, we might find an explanation of the phenomenon ; 

 but as this theory is open to such distinct disproval in other 

 cases as to have been universally abandoned, a reference to it 

 in this instance is inadmissible. 



If we suppose that bark produces bark, and wood wood, 

 we still are obliged to understand the existence of a continuity 

 of tissue between the part producing and the part produced. 

 Say that the cambium is the common matter, out of which the 

 wood and bark are both formed ; this substance is an exuda- 

 tion of the inner surface of the bark or the outer of the wood, 

 and is incapable of distinct separation from either. So that if 

 we suppose that this central axis was alive at the period when 

 the wood was formed above it, it is difficult to understand in 

 what way that organic connexion, which must have existed at 

 the period of the new formation, was subsequently destroyed ; 

 and if we suppose the axis to have been dead at that time, 

 there would be nothing left out of which the new wood could 

 be formed. Hence, it seems impossible to avoid the conclusion, 

 that the presence of a central axis, having no organic con- 

 nexion whatever with the parts surrounding it, is incapable of 

 explanation upon this theory. 



But if we take the opinion of du Petit Thouars as the basis 

 of an explanation of the structure of this specimen, none of 

 the difficulties connected with the other hypotheses will be met 



