200 TIMBER AND SOME OF ITS DISEASES, [chap. 



them, siinpl}^ contenting" myself here with general 

 truths which will serve to render clearer certain state- 

 ments which are to follow. 



It is possible to make two generalizations, which 

 apply not only to the illustration (Fig- 26) here selected 

 but also to most of our timber-trees. In the first 

 place, the cortical jacket, taken as a whole, consists 

 not of rigid lignified elements such as the tracheides 

 and fibres of the wood, but of thin-walled, soft, elastic 

 elements of various kinds, which are easily compressed 

 or displaced, and for the most part easily killed or 

 injured — I say for the most part easily injured, because, 

 as we shall see immediately, a reservation must be 

 made in favour of the outermost tissues, or cork and 

 bark proper, which are by no means so easily destroyed, 

 and act as a protection to the rest 



The second generalization is, that since the cambium 

 adds new elements to the cortex on the inside of the 

 latter, and since the cambium cylinder as a whole is 

 travelling radially outwards — i,e, further from the 

 pith — each year, as follows from its mode of adding 

 the new annual rings of rigid wood on to the exterior 

 of the older ones, it is clear that the cortical jacket 

 as a whole must suffer distension from within, and 

 tend to become too small for the enlarging cylinder of 

 rigid wood and growing cambium combined. Indeed, 



