162 VEGETABLE ORGANOGRAPHY. 



like manner, if one side of a tree has its branches more 

 exposed to the action of the air and light, the cor- 

 responding part of the trunk grows more than the oppo- 

 site. It is by the union of these two causes that all trees 

 in forests, or avenues, grow more on the external than 

 on the internal side. Such is the very simple explana- 

 tion of the excentricity of the pith, which, in reality, is 

 only due to the inequality in the thickness of the layers. 

 We will hereafter revert to what takes place where the 

 numbers of the layers are unequal on the two sides of 

 the tree. 



If all that I have said about the woody layers has been 

 attentively followed, it will be seen that each of them is, 

 during its first year, a kind of very elongated cone, which 

 surrounds the pith ; that during the second it forms a 

 second cone, which surrounds the terminal prolongation 

 of the pith, and which is prolonged at the base in such 

 a manner as to cover over the cone of the first year ; and 

 thus cone after cone is formed in succession, until the 

 destruction of the trunk. It evidently results from this, 

 that each cone, or woody layer, only increases during the 

 first year of its life ; and that it is afterwards covered 

 over by subsequent cones, and is, as it were, shut up by 

 them in such a manner as not to be able to lengthen or 

 thicken any more : it remains, after some years, in an 

 almost passive state, and does not seem any longer to 

 form part of the living organs of the plant. It results 

 from this state of things, that the woody layers serve suc- 

 cessively as coverings to each other ; and if one of them 

 has received any injury — as the action of frost, having 

 letters cut in its tissue, or cavities hollowed out in its 

 thickness, having nails driven into it, &c. &c. — all these 

 injuries, covered by subsequent layers, may be again 

 found after any number of years : experiments have 



