3 2 4 
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 States whose differentiation depends 
largely upon these characteristics. But in whatever variety of 
form, 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 Conditions Determined Xeromorphy and the Origin 
of Land Plants. 
The characteristic xerophilv 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 structure 
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 
