2l8 



THE CARBOHYDRATES 



Ritter and Fleck.* Lignification, therefore, is a progressive 

 change. Those cells which are destined to become tracheae 

 and sclerenchyma lose their living contents, all of which are 

 used up in the making of the encrusting substances. Those 

 which develop into parenchyma, on the other hand, retain 

 their hving contents notwithstanding the fact that their walls 

 may be considerably lignified. Not infrequently the older 

 wood ceases its normal functions, and passes over into heart 

 wood where the vessels may become repositories of sundry 

 waste metabohc products such as tannins, colouring matters, 

 and inorganic salts such as calcium carbonate and calcium 

 oxalate. Lignification gives the cell a greater power of re- 

 sistance to pressure, and a diminished power of resistance to 

 torsion.f 



The wood of gymnosperms differs from that of angio- 

 sperms in the fact that vessels are absent. Further, the xylem 

 of many gymnosperms is characterized by the presence of 

 resin ducts which are charged with resins and terpines which 

 form the source of terpentine and colophony of commerce. 

 It will be apparent from this brief consideration that the 

 analysis of wood gives very different results, according to the 

 nature of the material, its origin, and age. The following 

 table gives an approximate composition of the wood of 

 spruce : — 



53-55 per cent. 

 3-o6 



12-25 



30-00 

 2 00 

 i-oo 



The composition of hard angiospermic wood differs from the 

 above in generally containing slightly less lignin ; the amount 

 and nature of the hemicellulose content is also different. 

 Hard woods are characterized by containing considerably 

 more wood gum or xylan, the amount being anything from 

 15-24 per cent, whereas from 8-9 per cent is an average figure 

 for gymnosperms. On the other hand, hexosans are much 



* Ritter and Fleck : " J. Ind. Eng. Chera.," 1923, 15, 1055. 

 t Sonntag : " Ber. deut. bot. Gesells.," 1901, 19, 138. Leon : " Zeit. 

 Verein. deut. Ing.," 1918, 62, 341. 



Cellulose 



Hemicelluloses : Hexosans 

 Pentosans 

 Lignin .... 

 Fats and resin 

 Protein 



