12 



OF WOOD IN GENERAL 



such ceUular tissue, when its constituent cells are not more than 

 three or four times long as they are broad, being technically known 



as parenchyma. ^ . ^ ^ a. 



As we have aheady seen, in addition to its function ol conduct- 

 ing liquids, which necessitates these vessels or other conducting 

 tissue, as it is termed physiologicaUy, 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 

 thickened and hard. The elements of this mechanical tissue are 

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

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



S S F T F T 



C 



Sv 



M 



JS Ccs 



Fig. 8.— Longitudinal section of a fibro-vascular bundle m Helianthus. (After 

 Saclis ) , /v , . 



P, pith ; Si spiral vessels of protoxylem ; F, wood-fibres ; T, traelie<J3 ; C, cambium ; 

 8v, sieve-tubes , B, bast fibres ; E, endodermis ; Cx, cortex. 



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

 mechanical strength or resistance by undergoing a change known 

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

 stance known as Ugnin. 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 

 constitution is, however, as yet unknown. It is harder and more 

 elastic than cellulose, readily permeable by water, but not absor- 

 bent, not, that is, retaining the water. It is more soluble in acids, 

 such as chromic acid, than is cellulose, and is recognised by turning 

 brown when treated with Schulze's solution, a mixture of zinc- 



