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 
