LIGNIN 217 



memory the more salient facts of its structure. Xylem, or 

 wood, is not a homogeneous material but a tissue made up of 

 various elements which differ in their structure and function, 

 and which occur in varying amounts in the wood of different 

 plants. These structural units are the tracheae, the water- 

 conducting elements, which may comprise both vessels and 

 tracheides ; the fibres which have a mechanical function ; 

 and the parenchyma, the only living cells of the wood, and 

 which is mainly concerned with the storage of food, chiefly 

 in the form of starch and fat, and is in communication with 

 the outer tissues by means of the rays. These elements have 

 their origin in merismatic tissue and all, in the first instance, are 

 living cells with thin walls composed of cellulose and pectin. 

 In the course of their development into permanent tissue 

 elements, the walls of some of these cells may remain unaltered, 

 but in the majority of those plants which undergo an extensive 

 secondary thickening, or attain a large size, the cell walls 

 undergo a great change, a reinforcement of the original mem- 

 brane by the incorporation of a number of substances known 

 collectively as encrusting substances, the chief of which is 

 lignin. This lignification never occurs uninterruptedly over 

 the whole area of the wall ; pits, either simple or bordered, 

 are left for intercommunication between contiguous elements, 

 whilst in the first differentiated tracheae the lignification may 

 only occur on a relatively small area of the wall. The rate 

 of lignification is very variable, depending on the conditions 

 of growth and the specific physiology ; Burgerstein * obtained 

 evidence of its inception in cells but two days old. Beckmann, 

 Liesche, and Lehm.ann,f in an extensive study of rye, traced 

 the variation in the lignin content with increasing age ; they 

 found that lignin from young tissues contained a much lower 

 percentage of methyloxyl groups than that of older tissues, 

 and the fact that there is a considerable variation in the amount 

 of these groups in the lignin of heart wood, or duramen, and 

 sap wood, or alburnum, of the same tree has been shown by 



* Burgerstein : " Sitz. Kais. Akad. Wiss. Wien.," 1874, 70, i, 238. 

 t Beckmann, Liesche, and Lehmann : " Zeit. angew. Chem.," 1921, 

 34, 285 ; " Biochem. Zeit.," 1923, 139, 491. 



