184 MINNESOTA BOTANICAL STUDIES. 



Koochiching, where Slicta amplissima (Scop.) Mass, was also 

 frequent on hard-woods of the swamps. The other members of 

 the formation are in the main at least plants frequently seen on 

 higher ground and on other trees, and at present we can only 

 say as to adaptation that for some reason they find the cedars 

 to be especially congenial hosts, or are perhaps attracted also in 

 part by moisture. The discussion of the formation must not be 

 closed without mentioning two lichen parasites almost always 

 found in the formation on the Parmelia and another frequently 

 seen on the Pertusaria^ all helping to form one of the most fas- 

 cinating lichen communities known to the writer. The two on 

 the Parmelia are Biatora oxyspora (Tul.) Tuck, and Buellia 

 parmeliarum (Sommerf.) Tuck. The one on the Pertusaria is 

 Calicium turbinatum Pers. 



There are yet two lichen societies of the swamps to record, 

 following out observations begun at Henning and Bemidji. 

 These were in those places noted especially in the more com- 

 mon tamarack swamps, but in the present region the cedars are 

 more common and very similar societies were detected under 

 them and on the dead branches. Possibly it may appear to one 

 not especially familiar with lichens that we are doing violence 

 to the conception of plant associations in general in the forma- 

 tion of some of our lichen societies in the midst of other plants 

 and also in making so many subdivisions. However, the estab- 

 lishment of the conception of an exclusively licheno-series of 

 formations has seemed to be the only way to deal with the 

 problems at hand successfully. Possibly our last formation 

 above and the next two below, for the sake of less subdivision, 

 might be regarded as different strata of a single society ; but 

 the fact remains that the presence of the plant assemblage of 

 the list last above seems more dependent upon the occupation 

 of the swamp by cedars, while those of the two below are de- 

 pendent in the first instance mainly upon the presence of dead 

 coniferous wood and in the second upon the moisture of the 

 soil and more or less shade furnished by the trees. The method 

 pursued has at least seemed best in working out results in the 

 swamps as well as elsewhere, and it appears to the writer that 

 a very similar one, though perhaps couched in different terms 

 and having less of subdivision, must be followed by any one 

 who attempts to deal with the ecologic distribution of the lich- 

 ens of a region thoroughly. The two remaining formations of 



