168 MINNESOTA BOTANICAL STUDIES. 



for the Lake Superior region. True, the time spent in the ter- 

 ritory treated in this paper was nearly twice as long, but after 

 one has been in a limited area for some time, species new to the 

 region do not appear so often ; and the larger number of the 

 present list can scarcely be entirely due to the additional time 

 spent, especially when it is remembered that each locality was 

 studied carefully for the sake of distribution rather than merely 

 for species new to the state. However, previous knowledge of 

 the lichens of the state in general and their habitats, gained 

 through long-continued study in the field, made it possible to 

 work more effectually and added considerably to the whole 

 number collected and especially to the number new to the state. 

 So on the whole, it can scarcely be said that the portion of the 

 boundary covered in 1901 is richer in lichen species than the 

 Superior region to the east and southeast. Indeed, the latter 

 region has more diversity as to elevation and temperature with 

 scarcely less as to kinds of trees and other lichen-bearing sub- 

 strata and should be, and probably is, richer in lichen species. 

 The cold winds along the north shore of lake Superior add 

 materially to the number of northern forms found there, while 

 the influence of the smaller Lake of the Woods and Rainy lake 

 in this respect is less marked. For this reason, again, the lake 

 Superior area should be the more productive of lichen species. 

 Turning to the region whose lichen flora was recorded in the 

 sixth paper of this series, various portions of which lie 50 to 

 150 miles south of the present area, it may be confidently stated 

 that the former with its 215 recorded forms is considerably 

 poorer than the latter with about one third more recorded. 

 The greater richness in lichen forms in the present region is 

 to be accounted for partly by the fact that there are more rocky 

 substrata and, on the whole, more variety as to substrata. The 

 difference due to presence of rocks is apparent enough, when 

 we record that the present region, like the lake Superior area, 

 has nearly as many forms on rocks as on wood, while the terri- 

 tory farther south has only about half as many. Indeed, some 

 portions of the region now under consideration are doubtless 

 even richer in proportion of rock lichens than the lake Superior 

 area, for in the present paper, a close analysis of the region 

 about Rainy Lake City shows that there are fully ten per cent, 

 more rock than tree lichens. And it may be remarked in pass- 

 ing that the richness in rocky substrata, and more effective 



