680 MINNESOTA BOTANICAL STUDIES. 



The following formation was distinctly discernible in the low 

 woods about the tamarack swamps near Henning, the species 

 composing it being unusually abundant. From the frequence 

 of Pertusarias rare elsewhere in the region studied during the 

 summer, I shall name it as follows : 



PERTUSARIA LICHEN FORMATION OF TREES IN LOW WOODS 



(HENNING). 



Physcia obscura (EHRH.) NYL. 



Collema flaccidum ACH. 



Collema nigrescens (HUBS.) ACH. 



Leptogium myochroum (EHRH., SCHAER.) TUCK. 



Pertusaria velata (TURN.) NYL. 



Pertusaria pustulata (Acn.) NYL. 



Biatora glauconigrans TUCK. 



Biatora rubella (EHRH.) RAB. 



Biatora varians (Acn.) TUCK. 



I failed to find such a formation elsewhere in the territory 

 explored. The plants in the formation grow on the common 

 deciduous trees of the area, whereas at Bemidji and Red lake 

 the swamps were surrounded for the most part by pines. As a 

 whole the various adaptations of the plants are not difficult to 

 detect. It is a little peculiar that the species of Physcia, be- 

 longing to this damp formation, belongs to the section of the 

 genus having a well developed parenchymatous cortex rather 

 than to the one having a non-cellular cortex of closely packed 

 hyphae. The Cottemas with their non-cortical thallus and the 

 Leptogium with a cortex of a single layer of cells are, of course, 

 quite at home in such a damp habitat, as are the Biatoras with 

 their thallus devoid of cortex. The Pertusarias have a fairly 

 developed upper cortex. 



The last formation to be recorded is a scattered one detected 

 in shaded places at Bemidji, which may be designated as 

 follows : 



BIATORA LICHEN FORMATION OF MOSSES (fiEMIDJl). 



Pannaria languinosa (Acn.) KBR. 

 Biatora vernalis (L). Fr. 

 Biatora sphaeroides (DICKS.) TUCK. 

 Biatora hypnophila (TURN.) TUCK. 



