2 BOTANICAL GAZETTE [JULY 
on the basis of their relation to the present ecological conditions 
of their habitat, this has been a far more difficult matter and has 
not met with unanimity of opinion. A knowledge of the flora of 
a region and the floristic status of successive periods of time is 
indispensable, if for no other reason than to indicate the various 
conditions frequented by species or groups of plants. But the 
statistical method must be supplemented by an adequate study of 
experimental tests. The varying activity of plants as individ- 
uals and communities is of greatest importance scientifically and 
must be determined in the field under measured conditions. 
Various theories have been put forward from time to time as 
to the environmental relations of plants in bogs, but none of them 
can be said to have brought nearer a solution of this phase of the 
problem. The historical aspect of the question need not be dealt 
with here in detail. The more important theories are those 
advanced by the following writers: Kra~MaNn (19) regards low 
temperature and strong drying winds as the prominent factors 
in high northern latitudes; ScarmpeR (29) emphasizes humus 
acids in the soil, abundance of soluble salts and alkalies, and 
regards bog habitats as being ‘‘physiologically dry’’; LivinGsTON 
(22) suggests the presence of chemical substances not in direct 
relation to acidity of the soil as acting on the vegetation; WARM- 
ING (32) is inclined to look upon the presence of free humus 
acids as the weightiest cause; Frita and ScHRrOTER (13) correlate 
the conditions with low temperature and lack of aeration in the 
soil; while SCHWENDENER (30) and CLEMENTS (5) hold that the 
structural peculiarities are not at all related to present habitat 
conditions but are primitive peculiarities, which now remain unal- 
tered but were originally developed under different xerophytic 
conditions. Another explanation, that of the toxicity of the 
habitat, and its consequent physiological aridity and selective 
operation upon forms striving for occupancy, has been offered by 
the writer of this paper. This view has come from a more detailed 
investigation of the physical and chemical characteristics of bog 
soils and their physiological property (7, 8). It emphasizes the 
active participation of specific microorganisms and fungi, a view 
which correlates also very well with the unproductiveness of differ- 
