206 TIMBER AND SOME OF ITS DISEASES. [CHAP. 





 consequently we have a gradually increasing bark 



proper, formed of the accumulated cork-layers and 

 other dead tissues. 



A great number of interesting points, important in 

 their proper connections, must be passed over here. 

 Some of these refer to the anatomy of the various 

 " barks " the word " bark " being commonly used in 

 commerce to mean the whole of the cortical jacket 

 the places of origin of the cork-layer, and the way in 

 which the true bark peels off: those further interested 

 here may compare the plane, the birch, the Scotch 

 pine, and the elm, for instance, with the oak. Other 

 facts have reference to the chemical and other sub- 

 stances found in the cells of the cortex, and which 

 make " barks " of value commercially. I need only 

 quote the alkaloids in Cinchona, the fibres in the 

 Malvaceae, the tannin in the oaks, the colouring-matter 

 in Garcinia (gamboge), the gutta-percha from Ison- 

 andra y the ethereal oil of cinnamon, as a few examples 

 in this connection, since our immediate subject does 

 not admit of a detailed treatment of these extremely 

 interesting matters. 



The above brief account may suffice to give a general 

 idea of what the cortical jacket covering our timber 

 is, and how it comes about that in the normal case the 

 thickening of the cylinder is rendered possible without 



