$22 BOTANICAL GAZETTE [AUGUST 
the two are bog shrub and tamarack. To these might be added, 
depending upon its width chiefly, the bog sedge. This conclusion 
is justified both by a study of the vegetation and the physical 
conditions. It is believed that the data furnished at least point 
strongly toward the supposition that these zones are today xero- 
phytic habitats, even though they also prove that these same 
areas cannot long remain so. Numerous attempts have been 
made to explain the xerophytic structures in the plants found in 
these zones, but only slight reference need be made to them in 
this paper. 
Some have regarded the peat bog as a hydrophytic habitat. 
Thus WuitForD, after adding to other factors that of ‘insufficient 
aeration of the soil which prevents a healthy growth of the root 
system of land plants and also bars the presence of nitrifying 
bacteria,’ says that ‘‘these probably bring about xerophytic 
structures of plants so commonly seen in hydrophytic habitats.”’ 
He quite correctly regarded the bog chiefly as a hydrophytic 
habitat. In the area in which he worked also the true bog plants 
were not as limited in their distribution as in southern Michigan. 
It is quite probable that other conditions would enter into a 
detailed study of bogs in the northern part of the state (20). 
CLEMENTS (6) feels “‘that the current explanation of xerophy- 
tic bog plants, etc., is probably wrong, and that the discrepancy 
between the nature of the habitat and the structure of the plant 
is to be explained by the persistence of a fixed ancestral type.”’ 
SCHIMPER (15) attributes the xerophytic structures to the 
presence of humus acids in peat which impede absorption. Liv- 
INGSTON (12) has shown that any effect produced by humus acids 
must be chemical, as ‘‘bog waters do not have an appreciable 
higher concentration of dissolved substances than do the streams 
and lakes of the same region.” NILtson attributes differences 
in structure between swamp and bog plants to a difference in 
food supply, but this will not hold in this area. 
TRANSEAU believes from his observations and experiments 
that “‘in so far as southern Michigan is concerned (18, p. 36), the 
substratum temperatures prevailing in bog areas do not seem to 
be adequate to account for the presence or absence of bog plants 
