PLANT STRUCTURE AND COMPOSITION 187 



few per cent in some soft woods to 50 per cent or more in some conif- 

 erous species ('J";ible 8-1). Lignin is deposited in the interstices be- 

 tween the microfibrifs and the miceUar strands of cellulose and other 

 components to form a three-dimensional plastic which serves as a 

 cementing substance for the woven pattern of the other components of 

 the cell wall. In wood lignin is located chiefly in the middle lamella 

 with smaller amounts in the primary and secondary layers of the cell 

 wall. In annual plants lignin is found on the surface of the fiber and 

 appears to be less highly cross-linked than in wood. It can be much 

 more readily removed by the chlorination or sulfite process. The 

 question of whether lignin is chemically bound to the neighboring 

 carbohydrates or whether the linkages occur only as hydrogen bonds 

 remains unsolved. 



Lignin is an amorphous, yellow or brown substance of indeterminate 

 molecular weight. It contains approximately 63 per cent carbon, 6 per 

 cent hydrogen, and 31 per cent oxygen; the observed values will 

 vary somewhat with its origin and the method of separation. Lignin 

 contains a large proportion of aromatic rings with a high content of 

 methoxyl groups. Complete oxidation yields carbon dioxide and 

 water. If the conditions of oxidation are controlled, it is possible to 

 isolate a variety of aromatic aldehydes and acids as well as small 

 amounts of aliphatic monocarboxylic and dicarboxylic acids. Mild 

 oxidation of spruce wood lignin, as in the presence of nitrobenzene, 

 yields up to 25 per cent vanillin. 



Oxidation of lignin derived from hardwoods yields syringaldehyde 



CHO CHO 



CH3O I CH3O I OCH3 



OH OH 



vanillin syringaldehyde 



in addition to vanillin. Lignin has also been subjected to alkali fusion 

 and to dry distillation with the production of small amounts of a 

 variety of aromatic compounds. In addition, small quantities of non- 

 aromatic substances, such as formic, acetic, and oxalic acids, have 

 been isolated. 



The high carbon and low hydrogen contents mentioned earlier, and 

 the variety of benzenoid derivatives isolated from oxidized or fused 

 lignin, indicate the aromatic character. Many structural formulas have 

 been proposed for the compound, but conclusive evidence to support 



