Ill 



but with hardly any overlapping. Branches and lobes are usually 

 1 to 2 mm. wide, with margins crowded upward where they meet, 

 and breaking into mounds of whitish or blue-gray soredia up to 

 1 mm. or more in diameter. These in contrast to the brownish gray 

 or purplish gray upper surface, which is often rather dark, give a 

 conspicuously spotted appearance. The rounded tips are usually 

 depressed and somewhat saucer-shaped. The under surface is pale 

 at the tips, becoming black, with many similarly colored, sometimes 

 branched holdfasts usually shorter than 1 mm. The interior, seen 

 by scraping off the upper surface, is a dull yellow pith, not white 

 as in most Papery Lichens. 



Fruits rare, up to 1.5 mm. in diameter, black or frosted white, 

 with a thin, blackening rim. Spores 2-celled, brown, 17 to 28 by 6 to 

 10 microns. 



As Physcia sorediata rarely fruits in this region, all the charac- 

 ters noted above should be studied. The peculiar pattern made by 

 the touching margins and close-clinging, depressed tips, with the 

 uniformly dusky color, spotted by soredia, give an appearance 

 which, once well learned, is easily recognized, but a beginner must 

 compare this with Parmelia tiliacea (Group 6), Cetraria placorodia 

 (Group 5), and Physcia stellaris, none of which have soredia, and 

 with Pliyscia spcciosa, P. astroidea, P. caesia and P. obscitra, all of 

 which, though bearing soredia, have more space between the parts. 

 These other lichens all have white pith, and from them the yellowish 

 pith of P. sorediata is sufficient distinction where it can be seen, re- 

 quiring only a good lens and a little careful scraping with the knife 

 or fingernail. It must not be confused with the orange-buff pith of 

 P. cndococcinea, which is visible from beneath the lichen, or with 

 the bright blood-orange pith of P. endochrysea. The closely related 

 P. Frostii is smaller, paler, white within, and found only on rocks. 



Pliyscia Frostii. Frost's Blister Lichen 



Also called Pyxinc Frostii. Found on granite, shale and sand- 

 stone, usually on a vertical face of the rock, in shade. L'ncommon. 

 It forms very flat, close-clinging rosettes up to 3 cm. across, with 

 parts usually about 0.5 mm. wide. The aspect resembles a smaller 

 P. sorediata, but the color is pale gray with cream-color tips. The 

 parts also, instead of flattened, tend to be convex, and their margins 



