MINNESOTA BOTANICAL STUDIES. 



gradual change toward a lichen-flora characteristic of the north- 

 ern United States at the present time. No doubt the 8 northern 

 species now found on the igneous rocks are being rapidly re- 

 placed by the more numerous species, which are better adapted 

 to present climatic conditions. With the exception of the Neph- 

 roma, the 8 species persisting, all lichens confined wholly to 

 rocky substrata, or essentially so, and being therefore favorably 

 situated as to substrata, have persisted longest against un- 

 favorable climatic conditions and the onslaughts of the species 

 which are to-day surely replacing them. JVep/iroma, which 

 is arboricolous as well as rupicolous, furnished only a few small, 

 sterile specimens clinging to mossy rocks. Ephebe was seen 

 in one spot only, and, so far as I could ascertain, Umbi- 

 lica7'ia persists only in a few cool, damp or shaded spots. 

 The three species named above, not closely attached to the 

 rocks, would naturally succumb to unfavorable conditions sooner 

 than those named below, and all of the three former are sterile 

 and apparently just on the verge of extermination in the locality. 

 The other three species all grow closer to the rocks and are 

 all abundantly fertile. Biatora seems to be rare and is not 

 strictly crustaceous as are the other two. Buellia is the most 

 common of the 8 northern species and is one of two that would 

 be expected to persist longest because of its strictly crustaceous 

 habit. • Lecidea is as thoroughly crustaceous, but not so 

 common as the Buellia. It must be noted that the latter plant 

 shows locally the strong tendency to vary so characteristic of 

 organisms attempting to adapt themselves to change in environ- 

 ment. Doubtless this variation has aided the plant somewhat 

 in succeeding best of all the present or former more northern 

 species of the locality against adverse climatic conditions. 

 Whether or not the three forms of the species listed arose from 

 one in this locality has no particular bearing in the matter as 

 could be easily shown. Also the distribution of the three forms 

 is so little known that knowledge as to which particular form 

 is most common locally would not show whether it is one most 

 commonly persisting in temperate regions or not. 



It is interesting to note the time involved in the establishment 

 of the Arctic flora and the change from this to the present 

 essentially temperate flora. According to Professor N. H. 

 Winchell's views as to the recession of St. Anthony falls, the 

 final retreat of the glaciers from the region occurred about 8,ooo 



