284 MINNESOTA BOTANICAL STUDIES. 



the analysis, especially as to amount of illumination and the 

 roughness of ligneous substrata, it will be seen that lines can 

 not be drawn very closely without entailing an amount of 

 minutiae which would be confusing and therefore unprofitable. 

 With the above brief statement as to the main purpose of the 

 present paper, I shall begin the consideration of lichen forma- 

 tions with the most distinct ones with which I am acquainted, 

 viz., those of the Sioux quartzite at Pipestone. These forma- 

 tions are distinct because for most part removed from trees from 

 which lichens commonl}'^ migrate to rocks nearby, producing 

 tension lines and mixture of formations and because the few 

 young trees found, though large enough to bear the foliaceous 

 lichens which commonly migrate to the rocks, have apparently 

 been isolated from larger areas of trees from the beginning of 

 growth and scarcely bear a lichen of any kind. The rocky 

 substratum is for the most part horizontal and exposed to the 

 sun's rays. In a few places occur perpendicular rock expos- 

 ures which are more or less shaded by trees, overhanging rocks 

 or north exposure. A few ombrophytic lichens occupy these 

 spots ; but they are all strictly lithophytic species, none of them 

 having, for the reason stated above, migrated from trees as we 

 shall find to be the condition in a later analysis of other similar 

 formations. Below I give first the lichen formation of the hori- 

 zontal exposed rocks and second, that of the more or less shaded 

 and damp rocks. Lecanoras predominate in the formations on 

 exposed rocks, which may accordingly be named as follows : 



Lecanora formation of the horizontal exposed quartzite 



(Pipestone). 

 Parmelia olivacea (L.) Ach. var. prolixa Ach., C. 

 Parmelia conspersa (Ehrh.) Ach., C. 

 Physcia tribacia (Ach.) Tuck., C. 

 Physcia cassia (Hoffm.) Nyl. 

 Placodium elegans (Link.) DC, C. 

 Placodium vitellinum (Ehrh.) Naeg. and Hepp. 

 Lecanora rubina (Vill.) Ach., C. 



Lecanora rubina (Vill.) Ach. var. heteromorpha Ach., C. 

 Lecanora cinerea (L.) Sommerf., C. 

 Lecanora xanthophana Nyl., C. 



