CHAPTER II. 



ON THE GROWTH AND STRUCTURE OF TREES 

 (Continued). 



MOST writers upon the subject of the growth of timber 

 are agreed in ascribing the hardening of the inner layers 

 of the wood to the indurating action of the juices upon 

 the fibres and on the tissues, and thus far I have treated 

 of the process as carried on solely by this means ; but 

 another and a very different force is thought to be simul- 

 taneously exerted, which must at least accelerate the 

 change we have referred to. It will be remembered that 

 each year the sap, collecting between the last-formed 

 ring and the bark, congeals there, and forms a new layer. 

 This layer, as it solidifies, appears to exert a double 

 influence upon the tree, inasmuch as it exercises an ex- 

 pansive force upon the bark, thereby causing it gradually 

 to yield, while the resistance it offers, slight though it 

 may be, acts as a compressive force upon the whole of 

 the tree comprised within the circumference of the new. 

 layer. By means of this compression the interior layers 

 are rendered more dense, horny, and compact ; and such 

 portions of them as can no longer find room in their 

 former position are forced upwards, the elongation being 

 facilitated by the ascent of the sap through the vascular 

 system, and by the consequent deposit of additional 

 substance. 



We thus find the woody layers gradually assuming a 



