THE WOOD4. 



161 



are all; present, ' and fheii- relative quantity and mode- of grouping determine the 

 character of the wood ; which appears accordingly as a loose porous mass abounding 

 in vessels and trachei'des, or even chiefly parenchymatous, or, on the contrary, 

 as. a firm, dense mass of tissue chiefly composed of libriform fibres. Libriform 

 fibres, trache'ides, and wood-parenchyma form in ordinary cases the dense matrix 

 of wood .in which the vessels run; these usually possess far wider lumina, 

 and are often recognisable on the transverse section even by the unaided eye as 



FIG, 167. — Tangential longitudinal section through the wood ot Ailaitthtis ^tandu/osa. ^^ vessels ; 

 It medullary rays cut across ; ^ wood parenchyma ; t tracheVdes, 



punctate openings, and on the longitudinal section as more or less evident narrow 

 canals. 



The vessels, tracheides, and libriform fibres of the wood, as soon as they are fully 

 developed, contain air, and their walls are usually strongly lignified ; only the paren- 

 chymatous fibres {Ersatz-fasern and proper wood parenchyma) contain protoplasm 

 and products of assimilation, and possess thin walls, which are pitted in the usual 

 way and are often feebly or not at all lignified. 



Apart from a few still doubtful exceptions, the woody mass arising from the 

 cambium in the course of years is composed of -concentric layers,- which appear 



[3] M . 



