Page:Timber and Timber Trees, Native and Foreign.djvu/33

 CHAPTER II.

ON THE GROWTH AND STRUCTURE OF TREES

(Continued).

writers upon the subject of the growth of timber I 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 simultaneously 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 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 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