712 MINNESOTA BOTANICAL STUDIES. 



Also, from scattered statements in this paper, the inference 

 might be drawn that I should have given more prominence to 

 difference of substrata in accounting for the difference in num- 

 ber of lichens in the localities compai-ed. Minneapolis gains 

 6 species on the Saint Peter sandstone, which is not found at 

 Payette, and lacks 6 species, occurring at Fayette, because the 

 calcareous rocks seldom outcrop at the surface, and 5 species 

 because of scarcity of calcareous earth. Possibly some allow- 

 ance should be made for a probable slight advantage for Fay- 

 ette in number of granitic rocks, though Minneapolis has the 

 advantage in the per cent of species on these rocks. Of the 5 

 species gain for Fayette in the figures given above. 3 or 4, 

 about 75 per cent, could be expected to occur at Minneapolis if 

 the substrata were, present. We could add as many more 

 species for the possible advantage of Fayette in granitic rocks 

 as substrata and still only have a total difference of 7 species 

 resulting from difference in substrata. This would reduce the 

 advantage of Fayette to be accounted for by difference in at- 

 mospheric conditions to 37 species, or 24 per cent. Subtract- 

 ing this from the total difference of 28 per cent, leaves a 

 doubtful 4 per cent to be accounted for by lack of substrata at 

 Minneapolis. 



It may also be thought that I have not taken into account the 

 usual decrease in number of species in passing from warmer to 

 colder regions. The distance of about 150 miles from south to 

 north between the two localities compared is so small that lit- 

 tle difference in number of species could result, the difference 

 in mean annual temperature being between 2° P. and 3 ° F. 

 The smaller number of individuals at Minneapolis also tends to 

 prove that the difference in latitude has not helped to produce 

 the difference in number of species, as the decrease in number 

 of species, caused by colder climate, usually gives place to an 

 increase in number of individuals. If the difference in lichen- 

 floras were due to the above cause, northern species should 

 come in, to some extent, at Minneapolis, to take the place of 

 those found at Fayette, and not at the former place. Parmelia 

 olivacea (L.) Ach., Evernia prunastri (L) Ach., Getraria ciliaris 

 (Ach.) Tuck., and possibly Alectoria jubata (L. ) Tuck var. 

 chalybeiformis Ach. are more numerous, and occur on more 

 substrata at Minneapolis as a result of more northern location . 

 but not a species has come in! 



So far as moisture influences distribution of lichens, the 

 region along the Mississippi river should increase in richness 

 of species as we pass south in the state from Minneapolis. Yet 



