101 



forking branches less than 1 mm. wide. The upper surface is 

 straw-color or yellowish green, and sulphur-colored dust (sore- 

 dia) often edges the branches or forms large rounded masses on 

 the older parts. The under surface is dark brown to black. 



Fruits not frequent, up to 5 mm. in diameter, sometimes 

 much larger, plate-shaped or wavy, yellowish gray and waxy 

 looking, the rim thin and irregular, dusted with yellow soredia. 

 Spores 8 to 13 by 2 to 4 microns, but scarce. 



Since there are few small lichens with yellow soredia, Par- 

 melia ambigua is easily identified. Its form closely resembles 

 Cetraria placorodia, but the yellow tint marks it at a glance, and 

 distinguishes it from all species of Physcia. The black under 

 surface separates it from Cetraria pinastri and Parmelia centri- 

 fuga, while the masses of yellow soredia mark it from all similar 

 lichens except Cetrari pinastri, which is yellow beneath, and 

 Parmelia incurva, white beneath and found on rocks only. The 

 yellow and orange lichens in Group 9 are distinguished by hav- 

 ing orange or bright yellow fruits. 



Parmelia centrifuga. Ring Lichen. 



An alpine species of the north seen occasionally on quartz 

 and sandstone as far south as the summits of the Shawangunks. 

 Its trunks and branches, usually about 1 mm. wide, and vary- 

 ing but little except to widen somewhat at the tip, radiate from 

 a center which with age becomes crust-like, and often falls 

 away, leaving the lichen an irregular ring rather than a rosette 

 as much as 10 cm. across. The branches, except at the flattened 

 tips, are convexly rounded and covered with minute, warty 

 wrinkles. The tips are straw-colored or yellowish gray, but the 

 older parts usually darken as though dusted wath soot. New 

 grow^ths often cover the old in shingle fashion, and in doing 

 so, may swerve sharply to one side from the otherwise uniform 

 lines of radiation, making a peculiar and characteristic pattern. 

 The under surface is whitish with a few dark, root-like hold- 

 fasts. 



Fruits, as much as 8 mm. in diameter, but usually smaller, 

 are chestnut-brown, closely seated on the older parts, and with 

 a thin, pale rim. They are not likely to be seen in the New York 

 area. Spores undivided, colorless, 7 to 12 by 5 to 6 microns. 



In size and color, Parmelia centrifuga resembles P. ambigua, 



