PHYSIOLOGY OF TREES. 263 



We now come to the last hypothesis which 

 has been propounded to account for this obscure 

 process of vegetable life. It differs from all 

 others, by denying- that the new accretions are 

 formed from the generative property of the sap, 

 or that the cambium is a dilatation of the albur- 

 num of the former year, or that it is supplied 

 with fibrous matter entirely from the superior 

 buds ; but that the vital principle of the tree is a 

 distinct organ always situate between the wood 

 and liber, and that it is that component of the 

 system which is susceptible of and receives the 

 action and influence of the surrounding ele- 

 ments, occupying the place and appearing in the 

 character of cambium during its summer growth, 

 and, when this is over, reposing as a thin body 

 on the exterior surface of the new alburnum, and 

 within the new liber which has been thrown off at 

 the same time. While in this latter state, that is, 

 from the time when the growth is arrested in the 

 autumn till its recommencement in the spring, 

 it occupies a very inconsiderable space, appear- 

 ing on the transverse section only like a very 



