tort] BURNS—HURON RIVER VALLEY 123 
or their xerophilous structures. Experiments suggest, however, 
that farther north this factor is of prime importance’’; again, 
on page 37, ‘‘an examination of all physical and chemical data 
now available fails to account for the differences in the flora of the 
bog and swamp areas of this region. The most important factor 
is believed to be the physiographic history. Where the habitat 
dates back to Pleistocene times and has remained undisturbed, 
we find today the bog flora. Where the habitat is of recent origin 
or has been recently disturbed, we find the swamp flora, or a 
mixture of bog and swamp species.”’ 
DacuHNowskI (7, 8) believes from his experimental work that 
the condition which gives rise to xerophily and to zonation in bog 
plants “lies rather in the toxicity of the soil substratum, that is, 
in the production of unfavorable soil conditions brought about 
by the plants themselves.’’ This author further says that “if 
water transpired is replaced by bog water,” which would be the 
case to a limited extent during the summer when the water table 
was low and conditions for transpiration excellent, “‘the soils 
become more toxic.’ That is, the first effect of lowering the 
water table would tend to make the habitat more xerophytic. 
Further, ‘‘decrease in toxicity always follows aeration of the soil 
and drainage’’; that is, an increased lowering of the water table 
admits oxygen, decreasing the toxicity of the soil and making the 
habitat less xerophytic. With this is also associated an addition 
of humus which increases the capacity of the soil “for the adsorp- 
tion and retention of the toxins.” If the results obtained by 
DACHNOWSKI are correctly explained by him as due to bog toxins, 
they do not conflict with the data recorded in this paper. It is 
to be hoped, however, that his studies will be pushed farther. 
Davis, as has been pointed out earlier in this paper, believes 
that the bog is a xerophytic habitat, due to the drying of surface 
layers and the ability of peat to hold large amounts of water which 
are not available for the plants. The data given in this paper are 
in accordance with this view, and it appears to the author to more 
nearly cover conditions than any of the other theories advanced. 
It is not contradictory to the experimental work of the authors 
cited; this has been shown in the case of the work of DACHNOWSKI. 
