10 MINNESOTA BOTANICAL STUDIES. 



Superior down to the southern part of this county, and with 

 substrata abundant, they could do so in spite of unfavorable 

 climatic conditions. They have apparently failed to advance 

 as far as Taylors Falls, because of favorite substrata becoming 

 somewhat scarce, and an increase of unfavorable conditions as 

 to temperature and precipitation. I am aware that the glaciers 

 probably retreated slowly enough so that forests could spring 

 up and furnish substrata for the retreat of species driven south 

 in glacial times, before they would die out at the south on ac- 

 count of the return of warm climate, and that whatever northern 

 tree lichens exist in the pineries fifty miles north, could be ac- 

 counted for, wholly or in part, as having migrated from the 

 south. Yet I am quite convinced that there has been a circula- 

 tion of arboreous lichen-floral elements, between Lake Superior 

 and Pine county, in post-glacial time, which has not extended 

 to Taylors Falls, to any easily observable extent surely, though 

 conditions as to substrata are much more favorable for such 

 lichens to move southward from the lake than for the rock 

 lichens. 



Not a single species of northern lichen was found in the rock 

 crevices or soil studied. I have noticed how in regions recently 

 burned the soil becomes literally covered in places by lichens 

 of various genera in five to fifteen years, and there can be no 

 doubt that earth lichens took possession of the glacial drift rap- 

 idly after the retreat of the ice began. However, from the very 

 fact that lichens spring up rapidly on earth, the species charac- 

 teristic of temperate climate would the more quickly take pos- 

 session of the present limited amount of soil available for lichen 

 growth, and whatever additional amount that was available 

 when the strife began between arctic -and temperate earth lich- 

 ens, and the more rapidly kill out the northern species once in- 

 habiting the drift. 



A consideration of the statements made in the last two para- 

 graphs and various other portions of this paper points to the 

 conclusion that a study of the whole lichen-flora of the area 

 between Taylors Falls and Lake Superior is essential to a bet- 

 ter understanding of the problems herein considered. In the 

 next paper of this series, in which I shall consider the lichen- 

 flora of the Lake Superior region, I shall be able to show ad- 

 ditional reasons for the study of this territory. It is one of 

 rapid transition in lichen-flora, and after a study of the areas to 



