FINK—THE LICHENS OF MINNESOTA. 199 
lobes frequently closely imbricated toward the center and forming a continuous crust; 
beneath varying from pale brownish to black and when black having brown 
margins; rhizoids usually present and of the same color as the lower surface; apo- 
thecia common, subsessile, middle-sized, 3 to 11 mm. in diameter, the disk chestnut, 
concave, the margin crenulate or subentire; hypothecium pale or slightly colored ; 
hymenium pale below and pale or brownish above; paraphyses commonly simple, 
somewhat thickened, and frequently brownish toward the apex; asci clavate; spores 
ellipsoid, 8 to 11 » long and 4.5 to 7 p» wide. 
Occurs in all portions of the State. On igneous or metamorphic rocks, and rarely 
on wood. 
Common throughout the northern States, Alaska, and British America and farther 
south in mountains. Distributed in all of the grand divisions. 
15. Parmelia centrifuga (L.) Ach. Meth. Lich. 206. 1803. 
Lichen centrifugus L. Sp. Pl. 1142. 1753. 
Thallus in ours rather small, 3 to 6.5 cm. in diameter, but in European specimens 
reaching 10 cm., prostrate, composed of narrow, elongated, convex, crowded, and 
imbricated, centrally more or less rugose-plicate lobes, the central crust thus formed 
frequently falling away and leaving only the concentrically arranged periphery, 
straw-colored varying from greenish to yellowish; margins of the lobes sinuous, 
crenate, or incised; under surface whitish with darker rhizoids; apothecia small, in 
American and European specimens seen 2 to 3 mm. in diameter, said, however, to 
be middle-sized, sessile, the disk chestnut, concave, the margin subcrenulate; hypo- 
thecium pale or yellowish; hymenium pale below and pale or brownish above; para- 
physes quite distinct, plainly jointed, commonly simple, somewhat thickened at the 
brownish apex; asci clavate; spores ellipsoid, 7 to 10 » long and 4.5 to 5.5 « wide. 
Ours sterile and the apothecial and spore characters taken from other material. 
Once collected on rocks in the Misquah Iills. 
Commonly confined to arctic or subarctic regions or to mountains, but. found by 
Agassiz on the north shore of Lake Superior, In northern Europe and Asia as well 
as in North America, 
CETRARIA Ach. Meth. Lich. 292. pl. 5. f. 3. 1803. 
The thallus is usually more or less ascendant. It may be either fruticose or foliose, 
and when the former it may be cylindrico-compressed or more or less channeled. 
The strictly fruticose forms are usually alpine, and our Minnesota species, except 
Cetraria islandica, are foliose, and closely adnate or more or less ascendant. In 
Cetraria islandica there is, as in the more foliose and prostrate forms, a well-developed 
cellular cortex on all sides; but in this species the algal layer is more scattered than 
usual in ours, being diffused throughout the medullary tissue or forming interrupted 
layers beneath the cortex on both sides of the thallus, while in our other species the 
algal cells are to be found in a definite layer just below the upper cortex. The thallus 
varies greatly in color. 
The apothecia are disk-shaped or somewhat irregular in form, sessile and sub- 
pedicellate, terminal or marginal on the thallus, the disk differing from the thallus in 
color, The paraphyses and asci are rather short, composing a rather thin hymenium. 
The spores are hyaline, simple, rather small, subellipsoid to subspherical in form. 
Through the more erect forms the genus shows a close relationship to Evernia and 
indirectly with Usnea, while through the more foliose, less ascendant species the rela- 
tionship with Parmelia is close. 
Cetraria is represented in Minnesota by six species, of which Cetraria ciliaris is the 
only one generally distributed over the State. 
The plants occur on trees, old wood, rocks, and earth, 
Type species Cetraria lacunosa Ach. loc. cit. 
