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would hardly be thouglit Papery Lichens until examined closely. 

 Completely developed and fruiting rosettes may often be only 1 cm. 

 or less across, but often the individual rosettes merge in almost 

 uninterrupted masses a meter or more across and 2 or 3 mm. 

 thick. The narrower parts, especially when growing on rocks, may 

 be only 0.1 mm. wide, while on trees the lobes may broaden to 

 1 mm. or more, but are then commonly shorter than 1 cm. The 

 tips and margins tend to curl away from the foothold, dividing into 

 fine, lacy points dusted white. The upper surface is pale gray, often 

 covered centrally with a minute lacy growth which makes the 

 entire lichen appear white. There are no massed soredia, but 

 only fine dust. The under surface is pure white, and there are 

 spreading, white holdfasts often much in evidence. 



Fruits are frequent, up to 2 mm. in diameter, either blackish 

 brown or frosted white, with a pale gray, often crinkled rim. Spores 

 2-celled, blackish, 16 to 23 by 7 to 10 microns. 



Physcia tribacia, though variable, is one of our easy lichens 

 to learn because common everywhere, especially on roadside oaks. 

 Most other Papery Lichens small enough and pale enough to re- 

 semble it, do not lift their tips. P. teretiuscula has even narrower 

 parts, which cling close to the rock, and smaller spores. P. astroidea 

 tends somewhat larger, without the white points on tips and mar- 

 gins, and with mostly blackening holdfasts. P. tribacia must not 

 be confused with Cladonia caespiticia (Group 13), also plentiful 

 at the bases of trees, a dense mat of flakes, each usually no more 

 than 2 or 3 mm. long, but more than that in breadth, and instead 

 of spreading by trunks and branches, each flake springing individ- 

 ually from the foothold. Crocynia sonata (Group 14) should be 

 mentioned also, tiny and flat, marked with concentric zones of 

 whitish tips on dark gray, rosette-like, flaked crusts. 



Physcia teretiuscula. Small Blister Lichen 



Found occasionally on slate and shale, in rosettes usually less 

 than 1 cm. across, the parts closely pressed against the rock, 

 branches dividing to 0.1 mm. and smaller, with slender, pointed 

 tips touching the rock. The central parts pass into a confused 

 tracery with tiny granules and soredia, but all flat and thin, with 

 margins curling down to make the parts convex above, slightly 



