' 



SECT. II. COMPOSITE ORGANS. '220 



in the bark, and terminate at the line of union 

 between the graft and stock. * Also, if a portion 

 of the stem of a tree is decorticated so as to leave 

 the surface of the alburnum exposed to the air for 

 any considerable length of time, there is no farther 

 vegetation on that part of the alburnum. But if 

 the wound is not very large it will again close up, 

 first by means of the production of a new bark 

 issuing from the edges, and gradually narrowing 

 the extent of the wound ; and then by the pro- 

 duction of new layers of wood formed under the 

 bark as before. The new wood will not indeed 

 unite with the portion of alburnum that had been 

 exposed to the air ; but it will exhibit on a horizontal 

 section, the same traces of divergent layers as 

 before, extending from the bark in which they 

 originate to the lifeless surface of the old wood 

 within. It is evident, therefore, that the divergent whose 



layers are formed, not from the pith, but from the P inion i 



erroneous. 



proper juice descending through the channel of the 

 bark, and are synchronous in their formation with 

 that of the concentric layers through which they 

 pass. 



It seems indeed impossible that the divergent 

 layers should be an extension of the pith, at least 

 beyond the first or second year of the plant's 

 growth, whether on account of the gradual indu- 

 ration of the wood, or of its own natural diminu- 

 tion. For as the trunk increases in size the pith 

 * Phil. Trans. 1803. 



