THE STEM OF VASCULAR PLANTS. 175 



year it will be provided with two layers, and will be 

 enveloped at its base by the eleventh and twelfth layers 

 of the tree, and so on in succession. But the second 

 layer of the branch cannot reach so low as the first, for 

 it will find the place occupied by the eleventh layer of 

 the tree ; and for its part the twelfth layer of the stem 

 cannot surround the base of the branch so close as the 

 preceding, because the branch will have two layers 

 instead of one : it necessarily results from this, that 

 each branch, considered with regard to its base, or 

 inserted part, presents after some years a cone, the 

 point of which is at the place where it was originally 

 a bud, and the base is at the surface of the trunk. This 

 same branch, considered with regard to its projecting 

 part, also presents a cone, the summit of which is at the 

 end of the branch, and the base at the surface of the 

 trunk : the longitudinal section of a stem with branches 

 bears evidence of this structure, provided that it passes 

 exactly to the place where a branch takes its origin. The 

 base of a branch is, as we have seen, gradually imbedded 

 in the trunk like a nail, by the successive development 

 of the woody layers ; but as it enlarges at the same time 

 as the neighbouring layers, it gradually pushes them 

 back, so as to take the form of a cone. But if a branch 

 dies after a certain number of years, what happens ? Its 

 external cone, being exposed to the action of the air, is 

 destroyed; but its base, which is buried in the trunk, 

 becomes covered over with the new layers ; and as it 

 cannot any longer resist them, it is compressed and 

 squeezed by them on all sides. This is the origin of the 

 knots which are found in trunks ; and which are so 

 visible in planks of deal, for example. I have had 

 occasion to see trunks of the Fir, almost all the wood 

 of which had been destroyed by damp, except these 

 knots, or remains of abortive branches, which, on 



