330 PROCESS OF PEVELOPEMENT. CHAP. IV. 



gradually diminishes in diameter, till it is at length 

 totally obliterated, and its place supplied with 

 wood, as in the case of the aged trunk ; existing 

 only in the smaller branches, or in the annual 

 shoot. 



Conver- But how is the formation of the wood to be 

 pith into C accounted for, that now occupies the place of the 

 wood. p^h ? I n the chapter on the anatomy of the plant 

 it was observed that the pith of the young shoot is 

 surrounded by a set of longitudinal tubes, forming 

 in the aggregate a cylinder, in which it is invested 

 as in a sheath, which M. Mirbel designates by the 

 appellation of the Tubular sheath; but which I 

 have thought to be better designated by the ap- 

 pellation of the Medullary sheath; inasmuch as 

 the latter term suggests the use of the organ in 

 question, which the former term does not. But 

 by whatever appellation it may be designated, it 

 appears that the tubes of which it is composed do, 

 in the process of vegetation, deposit a Cambium, 

 which forms an interior layer that is afterwards 

 converted into wood for the purpose of filling up 

 the medullary canal. Such is the account given 

 by M. Mirbel,* of the origin of the wood occupying 

 the place of the pith in the aged trunk, which is 

 countenanced by the fact of the developement and 

 existence of longitudinal tubes that are to be found 

 even within the body of the pith, as stated also in 

 the above-mentioned chapter. 



* Traite d'Anat. et de Phys. Veg. liv. iii. 



