169 



Most of the representatives of this genus are fairly well charac- 

 terized ; some critical study is, however, necessary to settle disputes 

 in regard to many of the supposed " varieties " and " forms." 



They are almost cosmopolitan in their distribution. The major- 

 ity of species perhaps occur in the north temperate zone and live 

 upon rocks and trees ; some species show a preference for the 

 rougher barks, while others seem to have no choice. 



PLATE 44. 

 Parmelia perlata (L.) Ach. 



1. Plant natural size. 



a, apothecia ; b, lobe of thallus; c, soredial patches. 



2. Section of apothecium. 



a, thecium ; b and c, the two layers of the hypothecium ; d, upper 

 algal layer; e, colonies of algae scattered through the medullarv 

 layer; f, lower algal layer; g, lower cortical layer. 



3. Section of thallus. 



4. Paraphyses and spore-sac. 



5. Spores. 



6. Algae and haustoria. 



7. Cetraria Ach. Meth. Lich., 35. 1803. 



It is highly probable that this genus is phylogenetically derived 

 from Parmelia, or it may bear the same relation to Lecanora that 

 Theloschistes does to Placodium ; that is, it may be looked upon as a 

 fruticose branch derived from Lecanora or from Parmelia. 



The thallus-lobes are in all cases distinctly flattened. In the 

 majority of species a centric structure is evident. In some cases the 

 thallus lies quite flat and has a dorsiventral (bifacial) structure (C 

 juni-perina) . The upper and lower cortical tissues are well devel- 

 oped in both the centric and dorsiventral thalli ; in the former there 

 are, however, two algal layers, while in the latter there is only one. 

 Rhizoids are few or wanting, even in the horizontal thalli ; cilia 

 are, however, frequently present ( C. Islandica'). Many of the 

 Cetrarias also contain a high per cent, of acid crystals (cetraric 

 acid) deposited in and upon the hyphal cell-walls. The color of the 

 thallus is quite variable; it may be brown (C Islandica'), yellow 

 or yellowish ( C. juniperind) , or almost coal-black ( C. Fahhtnensis) . 



The apothecia are medium to large, sessile upon the upper sur- 



