12 MINNESOTA BOTANICAL STUDIES. 



(7) The migrating plants may have been in part or wholly 



killed out in some part of the series of migrations south- 

 ward in the Mississippi Valley, so that the present north- 

 ern floral element would have to flow in from the moun- 

 tains to the east and west, but more probably found 

 sufficient substrata and were not killed out in the south- 

 ward migrations in the valley. 



(8) The time involved in the change from arctic and sub-arctic 



to temperate flora is probably about 8,000 years. The re- 

 lative times occupied in the establishment of the northern 

 flora and the change to the temperature one can not be 

 estimated at present. 



(9) The absence of the northern floral elements from the sand- 



stone is due partly to the fact that many of the surfaces 

 of sandstone exposures are post-glacial. If some are as 

 old as the last retreat of the glaciers from the region, the 

 absence is still easily explained since the surfaces are 

 easily eroded and porous so that floral changes go on 

 rapidly on these rocks. 



(10) Lichens are not individually numerous on the sandstones 



because of this easily eroding nature which causes rapid 

 change and destruction ; yet a large number of species 

 become established in one place or another on them be- 

 cause lichens quickly gain a foothold. 



(n) The other 70 lichens of the rocks are not so character- 

 istically rock lichens and would migrate more easily and 

 rapidly, and the more quickly reach a locality and re- 

 place an established flora existing under unfavorable 

 conditions, because not confined to one substratum. 



(12) The rock lichen-flora of the locality is extremely rich be- 



cause of abundance of rocks, location in a river valley 

 where shade and moisture have been plentiful and geo- 

 graphically where the 8 northern species have persisted 

 to increase the number more characteristic of present 

 climatic conditions. 



(13) In substantiation of the method used to account for the 



present composition of the flora, I have attempted to show 

 that the absence of tree and earth lichens from the local- 

 ity tends to prove its correctness. 



In view of work already done at Taylors Falls and along 

 Lake Superior, the study of the region of rapid transition 

 in lichen-flora between is greatly to be desired. 



