14 



OF WOOD IN GENERAL. 



more than three or four times as long as they are broad, being 

 technically known as parenchyma, 



As we have already seen, in addition to its function of conduct- 

 ing liquids, which necessitates these vessels or other conducting 

 tissue, as it is termed physiologically, the stem has to perform the 

 mechanical function of bearing up a considerable weight itself, 

 its branches, leaves, etc. To enable it to do this, both xylem and 

 phloem are commonly accompanied by elongated elements, of 

 which the chief characteristic is that their walls are much 



E Cx 



FIG. 8. Longitudinal section of a fibre-vascular bundle in HeHanthus. 

 (After Sachs.) 



P, pith ; S, spiral vessels of protoxylem ; F, wood-fibres ; T, trachea; ; 

 C, cambium ; Sv, sieve-tubes ; B, bast-fibres ; E, endodermis ; Cx, cortex. 



thickened and hard. The elements of this mechanical tissue are 

 known as fibres, arid from containing them the bundles are often 

 termed fibre-vascular bundles (Figs. 7 and 8). 



The walls of cells, fibres, and vessels in the xylem acquire 

 mechanical strength or resistance by undergoing a change known 

 as lignification. This consists in their impregnation with a sub- 

 stance known as lignin. Lignin consists of the same three 

 elements as cellulose, viz. carbon, hydrogen, and oxygen, but in 

 different proportions, its percentage composition being 49 per 

 cent, of carbon, 6 of hydrogen, and 44 of oxygen. Its chemical 



