LICHENS OF THE MINNESOTA VALLEY. 283 



touched upon by Professor C. MacMillan.* It is interesting to 

 note that the strictly crustaceous Bucllia is the only one of the 

 more northern forms found in the state which persists as far 

 south as Pipestone. Indeed, its abundance here and records of 

 occurrence elsewhere well southward in low altitudes since 

 Tuckerman wrote lead to the suspicion that it may not be so 

 strictly sub-Arctic in distribution as I have supposed. It may 

 be added that the Buellia is the only one of these northern 

 species persisting in the valley, which was found on bowlders at 

 anv considerable distance from the large masses of Archean 

 and Algonkian rocks, which are supposed to have been exposed 

 continuously since the close of the glacial epoch, and that it 

 was only found once in very small quantity on a bowlder re- 

 mote from these larger masses. 



It has been my plan to introduce in each paper of the series 

 some feature regarding distribution which could be especially 

 well illustrated by the area under consideration. In the study 

 of the Minnesota valley and southwestern Minnesota I was 

 able to keep in mind a variety of ecologic factors and to pre- 

 serve the data necessary for their solution. This I had pre- 

 viously done in part for several areas in Minnesota and Iowa so 

 that in the present paper interesting and instructive comparisons 

 can be made. Leaving other questions, then, thus briefly 

 stated, I shall now pass to a consideration of the lichen for- 

 mations of the region, causes of their peculiar make-up, and 

 comparisons with similar formations within and outside the area 

 under consideration. 



Aside from the purely scientific interest of the analysis to 

 follow, it has a practical bearing, in that knowledge of the re- 

 lation between ecologic factors and distribution enables the col- 

 lector to predict in the field about what species of lichens he 

 may expect to find in a spot having a given set of environmen- 

 tal features. In the study species rarely found in the forma- 

 tions have not been considered when there appeared to be doubt 

 as to whether they were collected on their usual substrata, and 

 rarer varieties have been omitted when showing the same habi- 

 tat as other forms of the species. It will be readily granted 

 that the commoner forms which give character to the flora are 

 the ones which should receive attention in such a study. In 



* MacMillan, C., Observations on the Distribution of Plants Along the 

 Shore at Lake of the Woods. Minn. Bot. Stud, i : 967. 1897. 



