102 THE PLANT CELL WALL 



the more urgent problem, of stream pollution by such biologically 

 hazardous wastes. 



II. Coal and its Origins 



General Characteristics 



The fossil carbonaceous substances are sometimes divided 

 into leptobiolites, coals, and bitumens. The leptobiolites are highly 

 resistant ambers and pollens, and bitumens are derived from 

 marine humus. Coals are metamorphosed remains of terrestrial 

 vegetation, which may be derived from highly decayed gelatinous 

 vegetable matter (sapropel) or vegetable debris retaining to 

 a greater or lesser degree its biological organization (humus). 



Humic-sapropelic and sapropelic coals are richer in hydrogen 

 than humic coals, and contain relatively high proportions of re- 

 sistant materials such as pollen, wax, and resin. Sapropelic types 

 also include products of lagal origin. 



Vascular plant debris is generally accepted to be the precursor 

 of the great and familiar group of humic derivatives which include 

 peat, lignite, bituminous and anthracite coals. 



Coals may be classified mineralogically according to specific 

 gravity, hardness, color, luster, fracture (cleavage) and texture. 

 It will be more meaningful, from the viewpoint of origin, to examine 

 the petrographic classificiations of coals. Botanically, coals consist 

 of anthraxylon and attritus. The former is derived from woody 

 tissue and consists of smooth to lustrous homogeneous black bands, 

 stripes, and lenticular inclusions. Chemically, it consists of lignin 

 derivatives. Attritus is dull gray to nearly black, often striped 

 from macerated plant remains. It contains humic derivatives 

 together with resins, lipoidal substances (fats, oils, cuticular remains, 

 pollen coats) and minerals. These various constituents of attritus 

 form the basis for sub-classification according to their presence 

 and proportion. Lithological classification distinguishes vitrain, 

 clarain, durain, and fusion. Vitrain occurs in banded or lenticular 

 inclusions and has a woody microscopic structure. Clarain is 

 inhomogeneous, irregular, striated or streaked whereas durain is 

 finely granular and contains many pollen exines. Fusain is porous, 

 dull, and charcoal-like, consisting of a cellular network often 



