Fink : lichens of the lake superior region. 233 



Buellia petraea (Flot., Koerb.) Tuck. 



Buellia petraea (Flot., Koerb.) Tuck. var. montagnaei Tuck. 



Umbilicaria vellea (L.) Nyl. 



Nephroma tomentosum (Hoffm.) Neck. 



Pannaria epidiota Th. Fr. 



Stereocaulon paschale (L.) Fr. 



Cladonia amaurocrcea (Fl.) Schaer. 



Bseomyces aeruginosus (Scop.) DC. 



Heterothecium sanguinarium (L.) Flot. 



Taking into account the above table, we find that of a total 

 of 99 species and varieties generally distributed over the area 

 studied only lo, or one-ninth, are arctic or subarctic, and con- 

 sidering both of the last two tables we see that of 162 species 

 and varieties more or less widely distributed 16, or nearly one- 

 ninth, are arctic or subarctic. It has been stated that about 

 one-fourth of the species found only in one place are such north- 

 ern forms. Thus we find that the more general the distribution 

 of a series of plants in the area the smaller the per cent, of 

 northern species, and conversely the larger the per cent, of 

 temperate region species. In other words the prevailing species 

 are those characteristic of temperate regions, and as a whole the 

 rarer ones are the more northern floral elements. Since the in- 

 troduction of new species is commonly a more rapid process 

 than the complete extermination of others in a given region, the 

 existing conditions above stated seem to prove, as one would 

 naturally suppose, that the present lichen flora of the region is in 

 general of temperate region elements and that the more northern 

 elements of the flora are the persisting for most part in a few favor- 

 able spots. This supposition also explains the existence of the 

 northern species in isolated regiuns further south as I have done 

 for Taylors Falls. Professor Conway MacMillan has con- 

 sidered the spermaphytic flora of this region as a south-bound 

 one,* or at least that of the portion between the divide and lake 

 Superior. My observations here and at Taylors Falls do not 

 indicate that this is generally true of the lichens. However, 

 because of somewhat milder temperature, lower elevation and 

 perhaps more early retreat of the ice sheet in the western half 



* MacMillan C. Observations on the distribution of plants along the shore 

 at Lake of the Woods. Minn. Bot. Stud, i : 954. 1897. 



