Fink: LICHENS OF tjie lake superior region. 237 



Minneapolis, on the other hand, we find a Hchen flora, about 

 three-fifths of whose species and varieties are not found at Min- 

 neapoHs and about half of which are new to the state. The 

 region of rapid transition in lichen species lies between the 

 Minneapolis and Superior areas and has only been touched in 

 the study of its rock lichens at Taylors Falls. As stated in the 

 third paper of this series, this region is one of special interest 

 for tracing the distribution of species. The cause of the great 

 difference in lichen flora between Minneapolis and the Superior 

 region is scarcely due in any great measure to difference in 

 latitude since an almost equal difference in latitude to the south 

 of Minneapolis caused no appreciable difference in the flora. 

 Also, I have shown in this paper that in three localities lying 

 in the northern part of the region studied in the paper, viz. : 

 Snowbank lake, Ely and Gunflint, very few typically northern 

 species are found. These regions at the north of the area are 

 more closely related to the Minneapolis region as to lichen flora 

 than others 50 or 75 miles further south. The difference in 

 lichen flora between central and northeastern Minnesota seems 

 then to be due chiefly to three factors. The first is difference 

 in substrata. The limestones of the Minneapolis regions, as 

 well as the sandstone, are almost entirely wanting in northeast- 

 ern Minnesota, being replaced by an abundance of igneous or 

 metamorphic rocks. The conifers, which abound in the north- 

 ern part of the State, and which serve for substrata for quite a 

 number of species not found southward, form the other chief 

 difference as to substrata. Location in the valley of lake 

 Superior, where the region is shut off from warmer regions west 

 and north as well as south, is another factor that has caused 

 much of the difference in flora. It has been shown that the 

 number of arctic and subarctic species for a given elevation 

 decreases in passing southwestvv^rd along the lake. This I 

 suppose to be due not so much to difference in latitude as to the 

 fact that in the northeastern part of the region studied along the 

 lake the winds coming from the broader expanse of water, on 

 this largest American fresh water area, are rendered cooler than 

 farther down where the lake is not so wide. That the cold 

 winds are a factor is demonstrated by the occurrence of a large 

 proportion of northern species at the Grand Portage area, and 

 especially on the island which rises little more than 100 feet 

 above the lake, while such inland areas as Gunflint and Snow- 



