234 MINNESOTA BOTANICAL STUDIES. 



of the State, the temperate region lichens have no doubt driven 

 or followed the northern species farther north there and doubt- 

 less very few of the latter elements now exist in western Min- 

 nesota south of Lake of the Woods, or indeed anywhere in the 

 western half of the state. Since gaining possession of the north- 

 western part of the state these temperate region lichens have 

 doubtless been moving southward over the Mesabi range to 

 meet similar floral elements of a generally northward bound 

 lichen flora. The western half of the state remains to be ex- 

 plored for lichens, but the above statement as to the character 

 of the flora, based on observations recorded in this paper for 

 localities north of the Mesabi range and some knowledge of the 

 general character of the region, I regard as sufficiently 

 secure. 



Like the Taylors Falls region this one of course at one time 

 contained only arctic species, and the present more numerous 

 species characteristic of temperate regions have gained the 

 ascendency in quite recent time. However, the problems in- 

 volved in the struggle betwen the contending floral elements 

 do not force themselves upon the observer so strongly in this 

 larger area and must be dismissed with a much briefer state- 

 ment. As in the Taylors Falls region the persisting northern 

 species are largely lithophytic. This is shown in the following 

 exhibit of substrata for the 42 species : 



Arctic or subarctic lichens confined to rocks 25 



" " " " " " trees S 



" " " " " " earth 4 



" " " " " " dead wood 2 



" " " " " " dead wood and trees i 



" " " u " " earth and rocks I 



" " " " " " trees " " i 



The greater persistence of the lithophytic species is doubtless 

 due in part at least to the greater stability of the rock surfaces 

 and also probably in part to the fact that the arctic and sub- 

 arctic species became more thoroughly established on the rocks, 

 which were present for them to attack immediately at the close 

 of the Pleistocene before the advent of large trees and temper- 

 ate-region lichens. 



Thus far I have given a detailed account of habitat for the 

 northern species only. Of the whole 258 species and varieties 



