1 9 03J BOG PLANT SOCIETIES 405 



First. 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, Corn- 

 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. /) 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 all 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 



