Exogenous Tree. 477 



resembling one of those concentric zones characteristic of dico- 

 tyledonous trees. 



The connexion of the lower extremity of this shoot with the 

 trunk was a little below the ground line ; the shoot itself was 

 between four and five feet long; and throughout the whole of 

 this space there was, as I have before stated, no organic con- 

 nexion whatever between the surface of the shoot, and that of 

 the wood which overlaid it. 



The question which naturally arises out of the consideration 

 of this specimen is, in what manner the shoot could possibly 

 have been formed in the situation in which it was found. This 

 enquiry is so closely connected with the formation of wood itself, 

 that, before I attempt to offer any explanation of the specimen, 

 it is necessary to examine in review the most important theories 

 of the formation of wood in exogenous trees, which have, up 

 to this time, been advanced by different physiologists. These 

 may be divided into four classes : 1st, that the bark is pro- 

 duced by the wood ; 2dly, that wood is produced by the bark ; 

 3dly, that bark and wood reproduce themselves ; and, 4thly, 

 that neither the wood nor the bark produces the new matter 

 which is deposited upon them, but that the latter owes its origin 

 to the vegetation of the leaf-buds. 



The first of these opinions has been attributed to the Rev. 

 Stephen Hales, in whose very curious and useful work, called 

 Vegetable Statics, such sentiments are said to be discoverable. 

 I suspect, however, that there must be some mistake in this, as 

 I have not succeeded in meeting with any passage in that work 

 which can be said to indicate that such was the theory of the 

 author. He says, indeed, vol. i., p. 334, that he ' agrees in 

 opinion with Borelli, who, in his book, De Motu Animalium, 

 part 2, ch. xiii., supposes the tender growing shoot to be dis- 

 tended like soft wax, by the expansion of the moisture of the 

 spongy pith.' But it is not, perhaps, very important to inquire 

 whether he did entertain the opinion ascribed to him, as, if he 

 did, it has never been adopted by any succeeding writer, and 

 appears to be totally unsupported by evidence. 



The second opinion is that of Malpighi and Grew, the latter 

 of whom, in his Anatomy of Plants, 2d edit., book i., p. 114, 

 11, expresses himself thus : ' Every year the bark of a tree is 



