324 The Ohio Naturalist. [Vol. XI, No. 6^ 



certain point the fuel value or fuel ratio of coal can therefore be 

 determined by dividing the fixed carbon percentage by that of 

 the volatile hydrocarbons. A number of different kinds of coal 

 are recognized in the United vStates whose differentiation depends 

 largely upon these characteristics. But in whatever variety of 

 fonn, coal is derived from vegetation which grew in lowland, in 

 ponds and lakes in a manner as we find in sub-tropical swamps 

 and in peat bogs of temperate and northern regions today; it was 

 buried under successive layers of matter like itself, and of sedi- 

 ments such as sand and clay; thus protected from atmospheric 

 oxidation and subjected to gradually increasing heat, and the 

 pressure of overlying porous rocks, the vegetation became trans- 

 formed to the form we now use. The search for coal today is a 

 search for these ancient marshes, bogs and swamp-forests hidden 

 under layers of sandstone, shales, and drift (20) . 



What Coxditioxs Determixed Xeromorphy axd the Origix 



OF Laxd Plaxts. 



The characteristic xerophily of the carboniferous vegetation 

 has been interpreted by geologists (5) as indicative of a warmer, 

 moister atmosphere, more heavily charged with carbon dioxide 

 than at present. To the writer the facts are hardly consistent 

 with the external conditions assumed. The supposition that 

 xeromorphy involves factors of climate is not necessarily wrong, 

 but calls for a fuller consideration and comparison along with 

 additional factors, the character and magnitude of which is 

 capable of producing like results. A more satisfactory interpre- 

 tation of the phenomenon of xerophily would be found in the fact 

 that the present vegetation of undrained swamps and of bogs has 

 many of these xerophytic features none of which are correlated 

 with atmospheric influences only. The chief cause for both the 

 xerophily of the coal flora and the great accumulation of vege- 

 table matter is not to be looked for merely in climatic implica- 

 tions. High temperature and humid air promote in a high 

 degree decomposition. The great thickness of the deposits sug- 

 gests rather that the preservation of the debris was favored by a 

 temperate climate and by agents in the soil such as are involved 

 in the accumulation of peat today. Similarly the force of the 

 inference from the xerophytic aspect of the carboniferous veg- 

 etation — namely, the peculiarities of leaf size and leaf stiticture 

 for maintaining a balance between supply and loss of water — 

 gives additional support to the view that the plants encountered 

 adversities of soil-water content rather than of climate. A sat- 

 isfactory explanation of the phenomenon has been found in the 

 experimental investigations of the writer on the reduction action 

 and toxic character of bog water and bog soil (10), the results of 



