106 THE PLANT CELL WALL 



stances such as lignin, tannins, cutin, and the like are thereby 

 concentrated and rendered susceptible to further modification. 

 The secondary phenolic substances, are believed to be the principle 

 precursors of coal. Quantitatively, lignin assumes a place of greatest 

 importance in coal formation therefore. 



Experimentally, lignins can be transformed into products which 

 resemble coals in elementary composition and certain properties. 

 Bergius autoclaved spruce lignin which analyzed C 64.5 per cent, 

 H 4.5 per cent, or (CnH^O^,, with water at 340°C for 3 hr. 

 This hydrothermal treatment yielded C0 2 and a product analyzing 

 C 83.1 per cent, H 5.6 per cent. After fractionation, two coal-like 

 substances corresponding to (CioH 10 0) 2 and (CioH 6 0) v were 

 isolated. Bergius further claimed that "lignin" from bryophytes 

 and lycopods was also converted to coal-like products by hydro- 

 thermal treatment. It is reasonably well established that the concen- 

 trated lignin (and tannin) residues from cell walls give rise to humic 

 acids, and that these in turn are transformed into peats, lignites, 

 and the higher ranks of coal. 



The role of base exchange as a first stage process has been 

 claimed by Taylor, who has described the rapid carbonization 

 of vegetable matter at 3 m depth in the alkaline soils of Egypt. 

 Experimentally, the rapid coalification of wood and other plant 

 substances has been carried out by sandwiching these materials 

 between sand and wet sodium alumino-silicates. Under such ex- 

 perimental conditions, methane is entrapped, C0 2 disappears, 

 and the sand floor becomes alkaline and anaerobic. 



The secondary stage is concerned with the fate of the initial 

 residues. Among the many factors worthy of note are: extent of 

 exposure of vegetable remains before burial; burial time; depth of 

 burial; heat; pressure; and escape of volatile matter. A few examples 

 will serve to illustrate the manner in which these factors influence 

 coalification : 



(a) Fusains are richer in regions which allowed dry rot of plant 

 remains extending above water surfaces. 



(b) In a general way, older coals are of higher rank than more 

 recent deposits, presumably as a result of prolonged exposure to 

 metamorphic processes. 



