1903] BOG PLANT SOCIETIES 419 
driven further south. The climate being more boreal in its char- 
acter favored the bog plants, so that they became practically the 
only competitors for the low-ground situations. 
The preservation of the bog societies in poorly drained situa- 
tions down to the present time seems to be due (1) to the lower 
temperatures prevailing there, (2) to the sterile nature of the sub- 
stratum, (3) to the completeness with which the substratum is 
occupied by the bog plants, and (4) to the fact that most bog 
habitats are associated with lakes, whose basins must be entirely 
filled with débris, before the drainage conditions will be naturally 
improved and made more favorable for the coming in of other 
plant societies. 
To account for the xerophytic character of many of the bog 
plants, experiments now being carried on seem to indicate that 
differences of temperature between substratum and air is ade- 
quate. But the presence in many of our bog habitats of swamp 
species which show no xerophytic adaptations suggests that such 
xerophytic structures may be unnecessary under present con- 
ditions in ¢his region. 
SUMMARY. 
To summarize the results of this study, we may say that, as 
shown by their geographic distribution: 
1. The bog societies are typical of the colder portions of 
North America and are closely related to the bog societies of 
Europe and Asia. 
2. They show an optimum region of dispersal having a moist 
climate, subject to very great temperature extremes. Within 
this region the plants have a greater range of habitats and an 
increased physical development. 
3. As we go away from this center, either north or south, the 
first forms to show the effect of climatic change in diminished 
size and frequency of occurrence are the arborescent species. 
The species which extend furthest from this optimum region are 
herbaceous forms. 
4. The bog societies are normally related to the conifer for- 
ests in their development to a climax tree vegetation. 
5. Where surrounded by oaks and hickories, or in general 
