76 



THE PLANT CELL WALL 



we find the number of fractions and their intensity to be markedly 

 reduced (Table 4). The apparent loss of aromatic content may 

 involve many factors, but two changes in lignins are known to 

 be brought about by elevated temperatures (157-230°C) in the 

 absence of oxygen. First, the characteristic ultraviolet absorption 

 curve of lignins is altered, becoming more flattened and less 

 distinctive, and second, purified dioxane— and alkali-soluble lignin 

 becomes progressively less soluble with increasing severity of heat 

 treatment. At 230°C, for example, dioxane solubility is reduced 

 three fold in 20 min. Even at 100°C, lignin treated in air for 10 

 days undergoes some loss of solubility, a marked shift in absorption 

 maximum (280 m[i.->310 m\i) and loses some 80 per cent of its 

 absorbancy per unit weight. 



Table 4. 



Ultraviolet Absorbing Substances in Extracts 

 from Modern and Fossil Forms 



The effects of such elevated, but not extreme temperatures 

 suggest the importance of anaerobic pyrolysis in modifying the 

 fossil lignins. 



II. Non-Vascular Plants, Protista and Metazoa 



The traditional approach to the study of cell walls views them 

 as finished morphological entities external to the life processes of 

 their cells of origin, or as an inert framework for the support of 



