172 ANATOMY OF THE GYMNOSPERMS 



involved, that they not only proceed very slowly, but that it is 

 possible to find plant remains which present different stages of 

 the process, as represented by the various forms of peat, brown 

 coal, soft coal, and anthracite. Being determined by the with- 

 drawal of hydrogen and oxygen from the original tissues, these 

 alterations must arise very unequally in different parts of the 

 plant body, as determined by the character of the tissue involved 

 and the relative percentage of carbon originally present in the 

 cellulose substance. In the progress of such changes, gases 

 constitute some of the most abundant and conspicuous end 

 products. While under ordinary circumstances they may be lib- 

 erated continuously, they may be stored under favorable condi- 

 tions, to be liberated in great volume at a later period. Thus 

 it has been shown, as the result of recent observations (88), 

 that where plant remains accumulate in large quantities, sul- 

 phureted hydrogen together with the light carbureted and 

 phosphurcted hydrogen arise. The two latter, being subject 

 to spontaneous combustion, take fire upon coming in con- 

 tact with the air, and, setting fire to the associated sulphurated 

 hydrogen, an extensive conflagration may result. Phenomena of 

 this kind on a large scale rarely come within the observation of 

 man, but that such have been observed affords abundant ground 

 for the belief that many forest fires of obscure origin are to be 

 accounted for in this way. Thus, once more comparing the per- 

 centage composition of the principal cellulose modifications, it 

 is found that normal cellulose contains 44 per cent of carbon, 

 lignin about 62 per cent, while cork contains upwards of 74 per 

 cent. In accordance with this principle it will be found that 

 wood tissue becomes carbonized sooner than the softer parts of 

 the structure, which may already have disappeared through the 

 operation of decay, or the highly carbonaceous cork tissue of 

 the bark may be converted into a structureless mass of carbon, 

 while yet the less carbonaceous wood tissue is preserved in all 

 its details. It is thus possible, in a silicified wood, to recognize 

 and define the general limits of the bark by the carbonized layer 

 which oftentimes forms the outer portion of a fossil wood. 



