Fuik : LICHENS OF THE LAKE SUPERIOR REGION. 235 



listed in this paper, so far as observation showed, 83 are epi- 

 phytic, 80 lithophytic and 29 may occur on either trees or rocks. 

 Another 29 are epigean, 22 were found only on dead wood, six 

 are lithophytic or epigean, three are found on earth and dead 

 wood, three are parasitic on other lichens and two were found 

 on living and dead wood. The above analysis of substrata is 

 somewhat different from that used in the second paper of this 

 series for comparison of substrata at Minneapolis and at Fayette, 

 Iowa ; but when reduced to that form shows that the percentage 

 of lichens growing on rocks is somewhat higher than that for 

 the two areas farther south, while the percentage of those grow- 

 ing on wood is considerably lower. The larger proportion of 

 rock lichens in the Superior region is due to at least three things. 

 First, the more extensive exposure of rock surfaces, though this 

 is offset in part at least by the existence of three distinct kinds 

 of rock in the Minneapolis region, viz. : the igneous or meta- 

 morphic boulders, the limestone outcrops and the Saint Peter 

 sandstone. Second, the rock surfaces become warmer, each 

 day in the warm portion of the year, than the trees, because of 

 rapid absorption of heat ; and this doubtless favors lichen de- 

 velopment on rocks in this northern region. Third, general 

 moisture of much of the surface due to the fact that the rocks 

 are comparatively impervious to water, so that much of it col- 

 lects in lakes and swamps, favors good development of rock 

 lichens as compared with the region about Minneapolis. At 

 Minneapolis unshaded rocks bear very few lichens, but in the 

 Superior region rocks are well populated with them at all eleva- 

 tions and in all sorts of environment at or above the water line, 

 except where killed by fire. 



The total number of genera for the region is 39, while the 

 number for Minneapolis and Taylors Falls, so far as is known, 

 is 34. The whole number for Illinois, as given by Wolf and 

 Hall* is 40 and for Iowa as recorded by the writer in two papersf 

 is 38. This total number of genera for the Superior region is 

 seen by the comparisons with both larger and smaller areas 

 further south to be rather lar<re for a somewhat limited northern 



*VVolf, John and Hall, Eliliu. A List of the Mosses, Liverworts and Lich- 

 ens of Illinois. Bull. Ills. State Lab. Nat. Hist. 2 : iS-34. June, 1S78. 



fFink B. Lichens of Iowa. Bull. Lab. of Nat. Hist., State Univ. of Iowa 3: 

 70-88. Mr. 1895, and Notes concerning Iowa Lichens, Proc Iowa Acad. Sci. 

 5 : 174-187. 1897. 



