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 (heart-wood) to the indurating action of 

 certain secretions as they accumulate in the walls and 

 cavities of the fibres and other tissues, and thus far I 

 have treated of the process as carried on solely by this 

 means ; but another and a very different set of events 

 bring about the different degrees of hardness often 

 found between one part of the annual ring and another. 

 It will be remembered that each year the cambium, 

 developed between the last-formed ring of wood and 

 the bark, exerts its specific activity and forms a new 

 layer. This layer, as it is completed, and its elements 

 become firmer by the thickening of their walls, appears 

 to exert a double influence upon the tree, inasmuch as 

 it exercises an expansive 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 parts of the layers are rendered more 

 dense, horny, and compact. 



