GROWTH. 123 



(couche regeneratrice), by Mirbel ; from this cambium 

 is supposed to be derived the wood. 



182. The cambium is by some supposed to be depo- 

 sited by both the old wood and bark ; by others, again, 

 only by the bark, and that it descends gradually from 

 above downwards, just as we know the wood is depo- 

 sited from the leaves towards the root. The reasons 

 that may be adduced in favour of the secretion of this 

 matter by the bark, are the following: In the bark 

 of some trees, especially that of the Beech, are often 

 to be seen little nodules which cause a projection of the 

 bark externally ; these bodies have been particularly 

 observed by Du Trochet, who gives them the name of 

 embryo-buds; he considers them to have their origin 

 in the parenchymatous tissue of the bark : they have a 

 bark of their own, but which is united to that of the 

 tree, which, in some cases, may be distinguished by the 

 direction of its fibres ; as they continue to increase a 

 series of concentric woody layers are deposited in them 

 around a central cellular system or pith, they have me- 

 dullary rays, and in fact an appearance and nature of 

 an Exogenous stem ; by degrees the wood of the em- 

 bryo-bud comes into contact with that of the parent 

 tree, the bark of the bud being absorbed at the point 

 where these meet. In the Cedar of Lebanon, branches 

 have been seen proceeding from the summit of these 

 embryo-buds. 



183. According to Du Trochet, the buds in spring 

 have plenty of cambium, and as woody matter has been 

 deposited only in connection with the bark, for these 

 buds have no connection at first with the old wood of 

 the tree, so the cambium must be looked upon as pro- 

 ceeding from that structure. And further, that it not 

 only proceeds from the bark, but likewise is deposited 

 from above downwards, is supposed, by Meyen, to be 

 proved by the following circumstances. 



184. If in spring, when the buds have just burst, we 

 make two circular cuts, one a short distance from the 



