176 MINNESOTA BOTANICAL STUDIES. 



we find in the formations on some of the islands the northern 

 Lecidea goniophila Kbr., Lecidea platycarpa Ach., Buellia 

 badioatra (Fl.) Kbr. and Buellia geographica (L.) Th. Fr. 



As to the structural adaptations, they are very apparent. 

 First of all there is not a very large or loosely adnate lichen in 

 the society, much less a fruticose form. The two varieties of 

 Parmelia are forms having rather more closely adnate thalli 

 and thicker cortex than their relatives in the same species on 

 trees or more shaded rocks. Also the third Parmelia is rather 

 smaller and more closely adnate than its near relative, Parmelia 

 caper ata (L.), Ach., more commonly found on trees. Careful 

 comparative studies of the microscopic characters of these last 

 two have not been made, but such study would doubtless show 

 that the exposed-rock-inhabiting species has a cortex better 

 adapted to exposure. An adaptation in all the closely adnate 

 thalli of the formation is protection from wind and consequent 

 more rapid transpiration and even destruction by being blown 

 loose, which would surely come to the larger fruticose forms as 

 certain Cladonias in such positions. On the whole, the larger 

 the thallus of a given lichen in the present formation, the better 

 the cortex, the larger ones commonly having good cortex both 

 above and below, and the smaller usually at least a good upper 

 cortex. More definite statements regarding the structure of the 

 thalli of the lichens in the various genera may be found in 

 certain portions of the last paper of this series and need not be 

 repeated here. 



We may now pass most appropriately to the consideration of 

 another lithophytic lichen formation, designated as follows : 



MIXED LICHEN FORMATION OF SHADED ROCKS 

 (RAINY LAKE CITY). 



A. Naturally belonging to the rocks. 



Ramalina polymorpha (Acn.) TUCK. 



Ramalina calicaris (L.) FR. var. farinacea SCHAER. 



Theloschistes lychneus (NYL.) TUCK. ? 



Parmelia perlata (L.) ACH. 



Parmelia perlata (L.) ACH. var. ciliata (DC.) NYL. 



Physcia obscura (EHRH.) NYL. var. endochrysea NYL.? 



Sticta quercizans (Micnx.) ACH.? 



Pannaria microphylla (Sw.) DELIS. 



