328 COMPOSITE ORGANS. CHAP. II. 



described. The bark of the Fir-tree displays no 

 net-work ; the Liber being merely a thick layer of 

 parallel and contiguous threads, that seem to be 

 composed of a succession of oblong and united ves- 

 sels, and closely cemented together by a glutinous 

 parenchyma ; and the outer layers being merely 

 thick plates of indurated pulp separated by a thin 

 membrane that forms an epidermis to each. 



If the cortical layers are injured or destroyed by 

 accident the part destroyed is again regenerated, 

 and the wound healed up without a scar. But if 

 the wound penetrates beyond the Liber the part 

 destroyed is no longer regenerated. Or if a tree is 

 bent so as to break part of the cortical fibres, and 

 then propped up in its former position, the frac- 

 tured fibres will again unite. Or if a portion of 

 the stem is entirely decorticated and covered with a 

 piece of bark, even from another tree, the two dif- 

 ferent barks will unite. Hence the practicability of 

 ascertaining how far the Liber extends. And hence 

 also the origin of grafting, which is always effected 

 by a union of the Liber of the graft and stock. 



SECTION V. 



The Ligneous Layers. 



Of two THE ligneous layers, or layers constituting the 

 wood, occupy the intermediate portion of the 

 stem between the bark and pith; and are dis- 

 tinguishable into two different sorts concentric 

 layers, and divergent layers. 



sorts. 



