3 90 MINNESOTA BOTANICAL STUDIES. 



are well developed and were studied again during the summer 

 of 1902. Here, on the upper slopes of Mount Josephine, may 

 be observed almost pure formations of Umbilicaria muhlenbergii 

 (Ach.) Tuck, and higher up this mixed with the last two species 

 of the above list. Again, passing down to the more perpendicular 

 and shaded rocks, the second and third species come in more 

 and more, till in favorable spots we may find the two mixed 

 in the same formation or limited pure formations of one, usually 

 the second of the list. But even Umbilicaria muhlenbergii 

 (Ach.) Tuck, does not shun shaded habitats when not occupied 

 by other plants, and specimens exceeding one foot may be found 

 on the shaded talus of Hat point at Grand Portage. And it 

 must be taken into account that this is not one of the large spe- 

 cies of the genus. On the surface of one of the large talus 

 blocks of the point, figured in the Botanical Gazette,* Mr. C. 

 J. Hibbard and the writer observed at the shore of Lake Su- 

 perior one of these Umbilicaria societies in miniature with all 

 of the species of the above list growing on the area of the rock 

 about two feet square. Since these societies have not been pre- 

 viously studied in this series of papers, it has seemed best to 

 pass somewhat beyond the proper confines of the present paper. 

 The Umbilicarias like high rocks, but will descend nearly to 

 the water level along the shores where they thrive best in Minne- 

 sota. No doubt the adaptation lies in the fact that these rock 

 lichens, with unusually large thalli for lithophytic species, seek 

 the moisture of the lake winds. Also, the species being in 

 general arctic, subarctic or alpine forms, there is a temperature 

 relation as well, and the cooling effect of the winds of the larger 

 lakes influences their distribution materially. As is well known, 

 the Umbilicarias are attached to the rocks in an unusually se- 

 cure manner by a strong umbilicus. This protects them from 

 destruction by being blown away on the high and frequently 

 exposed rocks, though larger and less closely adnate to the sub- 

 stratum than most exposed rock lichens. Again, the thalli are 

 strengthened by an unusually strong cortex, which holds them 

 firmly against the rocks when dry and more easily broken. It 

 is also noteworthy that the lower cortex is usually thicker than 

 the upper and serves to hold against the upward tendency of 

 strong winds, as well as for support of the large centrally 

 attached thallus. It should be noted further that all of the spe- 



* Fink, B. Bot. Gaz. 1. c, Fig. 3. 



