THE STEM OF VASCULAR PLANTS. 181 



its nature. 3d, It is asked, in this theory, how the 

 cortical layers, which appear to be produced at the 

 same time as the woody layers, are formed ? But it 

 might be answered, that they have the same origin as 

 the woody layers, and are thus derived from the buds. 

 4th. It is observed, that if the buds of a branch of the 

 Plane-tree, or "Willow, be removed, and it be then 

 placed in water, it shoots out roots from the lenticels 

 before the visible development of new buds ; and it has 

 thence been concluded that the origin of roots has no 

 connexion with that of the buds. Du Petit-Thouars 

 thinks to answer this objection by remarking that there 

 exist latent or adventitious buds, which begin to be 

 developed, and cause the production of roots. 5th. An 

 experiment, which appears to me decisive upon this sub- 

 ject, is this : — If a circular notch, or an annular section, 

 be made in the bark of a tree, in such a manner as to 

 cut off all communication between the upper and lower 

 parts, it is clear that, at the end of a year, if it be the 

 buds which produce the woody fibres, it will have one 

 layer more above the section than below it, and this 

 layer will be formed of descending or longitudinal 

 fibres : that if, on the contrary, there only descends 

 from the upper part of the tree the nourishment 

 elaborated by the leaves, there wiU be the same number 

 of layers above and below the section ; but the layer of 

 the upper part, being more nourished, will be the 

 thicker, and that of the lower will be smaller and 

 thinner. But the experiment* has given this last 

 result in Exogenous trees; and one is compelled, it 

 seems to me, to conclude, that the woody layers are pro- 

 duced by the formation of fibres which do not proceed 



* This experiment has not perhaps been made with all the care that 

 could be wished ; and as it appears decisive for or against the theorj', it is 

 desirable that it shoidd be repeated. 



