288 MINNESOTA BOTANICAL STUDIES. 



bution whose explanation would be very difficult or impossible to 

 trace ; but it is quite remarkable that with this exception all the 

 plants found in the two formations sixty miles away should oc- 

 cur in this lichen formation also, especially since there could 

 have been no rocky connection between the two areas since 

 glacial times. It is not strange that the exposed granite lichen 

 formation at Granite Falls should be a much richer one than the 

 two exposed formations at Pipestone combined ; for it is a much 

 larger area, is connected with a limestone lichen formation and 

 an epiphytic, and a number of swamps and ponds furnish mois- 

 ture along the borders. Indeed the presence of ten of the 

 fourteen additions may be more or less satisfactorily explained. 

 These I shall proceed to consider seriatim. 



Physcia stellaris (L.) TUCK., var. apiola TUCK. a litho- 

 phytic variety of a species common on adjacent trees. 



Lecanora frustulosa (DICKS.) MASS. a northern lichen not 

 extending so far south as Pipestone. 



Lecanora subfusca (L.) ACH., var. alliophana ACH. a variety 

 of a species common on trees near by. 



Lecanora subfusca (L.) ACH., var. coilocarpa ACH. as the last 

 above. 



Lecanora cervina (PERS.) NYL., var. cinereoalba var. nov. 

 has not been seen outside the Minnesota valley. 



Lecanora calcarea (L.) SOMMERF., var. contorta FR. a lichen 

 migrating from the limestone near by. 



Rinodina sophodes (Acn.) NYL. found on trees of the region 

 and perhaps migrating from them. 



Rinodina lecanorina MASS. a very rare plant which, therefore, 

 very probably does not exist at Pipestone or was overlooked. 



Urceolaria actinostoma PERS. as the last above. 



BiatorarufonigraTucK. a northern form not extending so far 

 south as Pipestone. 



Though somewhat confusing another similar lichen formation 

 must be introduced here for comparison as follows : 



Lecanora formation of exposed quartzite (New Ulm). 

 Parmelia conspersa (EHRH.) ACH., CTS. 

 Physcia caesia (HOFFM.) NYL., CTS. 



Placodium cerinum (HEDW.) NAEG. and HEPP. var. sideritis 

 Tuck., CT. 



