Fink: lichens of the Minnesota valley. 279 



less accessibility of rocky substrata, required more time. The 

 former area furnished 151 species and varieties and the latter 124. 

 A brief analysis of the causes of the advantage in favor of the 

 former region can be best made by a consideration of the sub- 

 joined table, giving the various substrata for both localities with 

 the number of lichens most commonly found on each. 



A complete analysis introducing per cents as was made in a 

 former paper is riot necessary since general likeness except for 

 trees is apparent in the table. The difference in richness then 

 is due mainly to absence of large areas of trees at Granite 

 Falls. The slight differences in the other three items in the 

 table is doubtless due to difference in moisture, the precipitation 

 .being 30.53 inches annually at Mankato for three years for 

 which I could get data and 21.83 inches annually at Granite 

 Falls for five years for which data were obtained. Difference 

 in moisture doubtless also accounts in small measure for the ad- 

 vantage of the Mankato area as to arboreal lichens. 



As to rocky substrata favorable to lichen growth little can be 

 definitel}'^ given by way of comparison. As to kinds of rocks 

 Mankato has an advantage in having the sandstone which is 

 wanting at Granite Falls, and also in the great masses of lime- 

 stone which are replaced at Granite Falls only by the calcareous 

 drift pebbles and a few bowlders. Yet these two advantages are 

 probably quite overcome by the great masses of exposed gran- 

 ite at Granite Falls, not replaced at Mankato in any way, since 

 granitic bowlders are equally abundant in both places. 



Comparing the Mankato vicinit}' with Minneapolis and with 

 Fayette, Iowa, two areas compared in a former portion of these 

 studies, we find that it has a much richer lichen flora than the 

 former region which gave only 113 lichen forms and probably 

 nearly as rich as the latter which gave 157 lichens which one 

 could expect to find in a study of limited duration. 



Minnesota has now furnished more lichens than an}' other 

 state in the Mississippi Valley, having 351 species and varieties. 

 Illinois with 249 lichens being next in order. Yet the fact that 



