1 903] BOG PLANT SOCIETIES 4 I 9 



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 (i) 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 debris, 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 this 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 



