April, 1911.] 
The Ancient Vegetation of Ohio. 
3 2 7 
whatever nature checked the activity of the roots of plants and 
depressed their transpiration. The striking similarity of the 
aerial shoots of the carboniferous plants to those of modem times 
in bogs and undrained swamps restrain one, therefore, from 
assuming that the atmosphere differed greatly in temperature and 
humidity, or was different in the chemical constituents from what 
it is now. There may have been moderate variations in the 
carbon dioxide content of the air, but this would require experi- 
mental proof upon bog plants and the groups of plants similar to 
those which lived in carboniferous times, the scouring rushes, the 
lycopods, ferns, cycads and gymnosperms, to assign its limits. 
The statements in current literature as to the strengths of that 
gas which green plants can endure are conflicting (6), and call for 
further work in the field and in the laboratory. 
The consideration of these facts leads to another point — the 
inevitable conclusion that the form characters and the funda- 
mental resistance to drought and dessi cation distinctive of xero- 
phytic plants whether in bogs or deserts must have made their 
appearance within early geologic time. They are not of recent 
development (15). The climate of northern America has under- 
gone oscillations between periods of maximum aridity and max- 
imum precipitation and humidity, with extreme variations in 
temperature during and following the several glacial periods; the 
amplitude occupying periods of perhaps many thousands of 
years. Variations in climate so wide apart indicate an almost 
complete change in the character of the flora during the geologic 
periods. The xerophytic features which characterize bogs and 
deserts are not to be taken, therefore, as having come about by a 
direct and continuously increasing edaphic or climatic aridity. 
Aside from the question as to the methods and the activating 
conditions in evolutionary development, it seems certain that the 
origin of xerophytic forms is not one of recent development in 
the vegetable kingdom but must have been concomitant with the 
diastrophic and gradation processes of the great geologic periods. 
The great floral evolutions of geologic history were principally 
one of growth-form, physiognomy, and functional behaviour, and 
not of floral structure alone. Water has always been the most 
important of all the life relations in the environment of plants. 
In the early types of gametophytic vegetation it remained neces- 
sarily of greatest importance for the movements of gametes in 
effecting fertilization and for dissemination. The luxurious 
development of these forms in the ancient areas of low lying land 
became checked in the stress of aridity encountered with the 
accumulation of their debris. With the origin and the develop- 
ment of the sporophytic types of vegetation, which were from the 
first less dependent upon free water, the prolongation of vegeta- 
tion activity enabled the plants to occupy the areas with greater 
