182 MINNESOTA BOTANICAL STIDIES. 



lichens of cedars and tamaracks like the cedars better, but those 

 of the above list are those that are at least fully as much inclined 

 to the tamaracks, and may be regarded as making up a definite 

 formation in pure tamarack swamps ; though it must be admitted 

 that the results are most confusing in the presence of cedars, 

 and indeed so much so that I am inclined to believe that, had I 

 first encountered a mixed swamp for study, I might have seen 

 a cedar-tamarack formation instead of two distinct ones. How- 

 ever, the conclusion that there are two is secure, and it may be 

 remarked in passing that for purposes of study of either of the 

 formations it matters little whether one enters one of the vast 

 swamp regions or a small swamp a few rods in diameter. And, 

 indeed, one is surer of his results if he confines himself to what 

 may be called a small society rather than to attempt the study 

 of a widely extended formation. In the small swamp one is 

 impressed with the fact that the characteristic lichens of the 

 formation seem to have entered from all sides from less favor- 

 able habitats or substrata, and they are more easily studied on 

 the small area. And the species may be found in nearly as 

 large numbers in the small as in the large swamp. It may 

 well be added in passing that the above principle in the study 

 of lichen societies has impressed the writer in the investigation 

 of lichen formations in general, a*nd it usually seems to matter 

 little whether the small formation is near other similar ones or 

 far removed and isolated. The adaptations of the above re- 

 corded formation have been dwelt upon in the sixth report and 

 need not be repeated here. So we may pass at once to the 

 formation of the swamp cedars. 



Sticta Pulmonaria Lichen Formation of Cedar 

 Swamps (Boucherville). 



Cetraria ciliaris (Ach.) Tuck. 



Cetraria lacunosa Ach. 



Cetraria aurescens Tuck. 



Parmelia borreri Tuck. var. rudecta Tuck. 



Pyxine sorediata Fr. 



Sticta pulmonaria (L.) Ach. 



Sticta amplissima (Scop.) Mass. 



Sticta fuliginosa (Dicks.) Ach. 



Nephroma helveticum Ach. 



Nephroma laevigatum Ach. 



