1903] BOG PLANT SOCIETIES 405 
Furst, What relation do these bog societies bear to the sur- 
rounding vegetation of oaks and hickories as they occur in Ohio, 
Indiana, and southern Michigan ? 
Second. How can we account for the presence of bog societies 
and swamp societies (or mixtures of the two) when found in adja- 
cent areas, having similar drainage and soil conditions ? 
PRESENT DISTRIBUTION OF BOG PLANTS, 
In order to obtain a better understanding of these questions, 
data on the geographic distribution of bog plants were collected 
and maps drawn. It soon became evident that the number of 
species would have to be limited and that only those which are 
characteristic of these situations across northern North America 
could be considered. The number was finally reduced to fif- 
teen. Beginning with those which first find a foothold in such 
depressions and continuing in their approximate order of advent, 
they are: Menyanthes trifoliata, Dulichium arundinaceum, Com- 
arum palustre, Scheuchzeria palustris, Eriophorum polystachyon, 
Drosera rotundifolia, Sarracenia purpurea, Oxycoccus Oxycoc- 
cus, Chiogenes hispidula, Andromeda Polifolia, Chamaedaphne 
calyculata, Ledum groenlandicum, Kalmia glauca, Betula pumila, 
and Larix laricina. 
Finally the accompanying map (fig. 7) was drawn by super- 
imposing the areas of North America in which each of these 
plants commonly occurs. In the course of its construction some 
interesting points in geographic distribution came to light. The 
dark area extending from the Atlantic to the Mackenzie basin 
represents a great center throughout which al] these plants 
appear in most bogs. The lighter shaded areas north and south 
represent territory in which only a majority of the forms occur 
in the average bog, while in the lightest shaded portions only a 
minority of them are to be found. 
CLIMATE OF THE OPTIMUM REGION. 
The region of optimum distribution is limited by certain 
climatic barriers. On the southwest its limits coincide closely 
with those of the forests. Beyond this area the relation between 
rainfall and evaporation makes the conservation of water in 
