124 GROWTH. 



other, round a stem or branch, and remove the ring-like 

 piece of bark between them, at the same time clearing 

 the exposed surface of the wood which will be thus left 

 bare, so that no portion of the internal layer of the bark 

 shall be left upon it, we shall find that by the expiration 

 of summer the end or surface of the upper cut is very 

 much swollen, whilst that of the lower is scarcely en- 

 larged at all. If the branch is now cut across between 

 them and examined, above the upper cut we shall find 

 a deposition of woody matter ; beneath the lower one, 

 no such deposit will be seen ; which is also the case 

 in regard to the surface of the wood which was ex- 

 posed. 



185. Leaving these considerations for a moment, we 

 may remark that the essence of the theory we are here 

 intending to illustrate is, that the new woody matter is 

 derived from the cambium, and that this matter has its 

 origin in the formative juices descending from the leaves 

 towards the root. The objections to this will be spoken 

 of afterwards. 



186. In 1809, Du Petit Thouars, in an "Essay on 

 the growth and diameter of the trunks of Dicotyledo- 

 nous trees, &c.," proposed a theory upon the origin of 

 wood, which has many supporters, and which, accord- 

 ing to them, is the only one that can explain the for- 

 mation of this structure throughout much of the vege- 

 table kingdom. 



187. This theory is that buds upon a tree are not 

 materially different from the seed of a plant, and that 

 like them they can evolve roots or radicles ; these are 

 developed from the base of each bud, and proceed down- 

 wards with considerable rapidity between the bark and 

 the wood, and connect themselves with the roots of the 

 buds below them. These roots are nothing less than 

 the fibres out of which the woody layer is afterwards 

 formed, and which are nourished by the cambium or 

 formative juice from the bark. 



188. Supported as this view of Du Petit Thouars is 

 by many circumstances, it fails elsewhere. If a scion is 



