24. USNEACEAE 



Thallus fruticose, erect, or pendulous, usually radial, sometimes dorso- 

 ventral, differentiated into a typical plectenchymatous cortex surrounding 

 the algal layer and medulla, the medulla being cottony, cobweb-like, solid 

 or hollow, attached to the substratum by a holdfast or scattered rhizoids. 

 Apothecia circular, marginal, subterminal, or terminal, sessile or subpedi- 

 cellate; proper margin absent, thalline margin present; asci 1-8-spored; 

 spores usually hyaline, rarely brown, nonseptate, 1-3-septate or muriform. 



Alga Protococcus. 



A. Medulla cottony. 



B. Medulla solid cotton. 



C. Thallus soft and flaccid, some shade of grayish-green. ...57. Evernia 

 C. Thallus stiff and rather wiry, chestnut brown to black. 



60. Cornicularia 

 B. Medulla not solid cotton, thallus rather stiff, with a strong central 



medullary cord 62. Usnea 



A, Medulla stiff or cobweb-like. 



D. Apothecia unknown, podetia hollow, always light colored, usually 



white 63. Thamnolia 



D. Apothecia rather common, podetia not present, 



E. Several definite medullary cords. 58. Letharia 



E, No definite medullary cord. 



F. Thallus filamentous, wiry or hair-like, center often hollow, spores 



nonseptate 59. Alectoria 



F. Thallus narrowly membranaceous, cartilaginous, stiff, often with 

 a flattened appearance, especially where it is attached to the 

 substratum, spores 1-3-septate 61. Ramalina 



57. Evernia Ach. Lichenogr. Univers. 84. 1810. 



Thallus gray or greenish gray, fruticose, tufted, or shrubby, erect or 

 prostrate, dorsi-ventral, fiaccid or somewhat stiff, much branched, often 

 matted and twisted, with a thin, irregular, plectenchymatous cortex, en- 

 tirely surrounding the algal layer and cottony medulla, composed of loosely 

 interwoven hyphae, attached to the substratum by a holdfast. Apothecia 

 medium to large, circular, terminal or marginal, sessile or subpedicellate, 



(147) 



