THE STEM OF VASCULAR PLANTS. 157 



strata badly nourish the corresponding parts ; and, con- 

 sequently, these have the layers thinner, and they 

 remain a longer time before they attain their complete 

 hardness. 



All workmen know very well that the alburnum is 

 less solid than the wood, and take care to separate it 

 from the latter when they use it for building purposes, 

 &c. BufTon, who performed with Duhamel some im- 

 portant experiments upon this subject, found that in 

 the Oak the difference of solidity of the alburnum and 

 the wood, is as six to seven. But the principal cause for 

 which the alburnum is carefully rejected from the wood in 

 building, is that on account of its looser tissue it is more 

 liable than the latter to be affected by moisture, worms, 

 and insects. We often find stakes placed in wet situa- 

 tions, with the alburnum either entirely decayed, or per- 

 ceptibly changed, while the wood is still very sound. In 

 order to remedy this inconvenience, Duhamel and Buffon 

 proposed, from their own experiments, to rind trees a 

 year before they are cut down ; by this the tree is pre- 

 vented from forming a new layer of the alburnum, and 

 the nourishment which would have been employed in 

 developing this new layer, being distributed to the 

 layers of alburnum already formed, causes them to 

 become almost as hard as perfect wood. This method 

 ought to be practised with timber destined for ship-build- 

 ing in particular, because this hardened alburnum is less 

 liable to be attacked by the Teredo ; it is, however, rarely 

 practised ; and it is asserted that it has the inconvenience 

 of rendering the wood more brittle. 



If we except the medullary part, each layer, whether 

 of the wood or alburnum, is composed of porous or 

 striped vessels, intermixed with elongated cellular tis- 

 sue ; this is so much the more elongated the harder the 

 wood is destined to become. The vessels are in general 



