170 



face of the thallus, but in the majority of species are few or wanting. 

 The disk is quite thin, margin somewhat torn or crenate, with a 

 brown shining epithecium. The colorless hypothecium is divided 

 into two well-marked more or less cortical layers. The paraphyses 

 and spore-sacs are very short; the spores small, otherwise as in Par- 

 melia ; the thecium of Cetraria resembles in all respects a reduced 

 thecium of Parmelia. 



Soredia are not of such frequent and plentiful occurrence as in the 

 preceding genus, which gives the thallus a somewhat smoother 

 aspect. The thallus is also more brittle than in Parmelia, due to 

 the greater development of the cortical layers, with a corresponding 

 reduction of the medullary tissue. 



Most of the Cetrarias are northern ; they occur upon trees, soil 

 and rocks of the higher altitudes and latitudes ; their distribution 

 east and west seems to be about uniform. Most of the European 

 forms occur in North America, but a few American species do not oc- 

 cur in Europe. 



PLATE 45. 

 CETRARIA JUNIPERINA (L.) Ach 



1. Portion of the thallus natural size. 



2. Section of apothecium. 



3. Section of thallus. 



4. Paraphyses and spore- sac. 



5. Spores. 



6. Algae. 



8. EVERNIA Ach. Lich. Univ. 84. 1810. 



This genus has very likely been derived from Cetraria, We have 

 here a markedly higher differentiation of the fruticose type ; the lobes 

 have become much narrowed and in the majority of species are 

 somewhat cylindrical and as a natural consequence show a radial 

 arrangement of the tissue-elements. 



Branching of the thallus is often dichotomous with an acropetal 

 decrease in the size of the lobes (.Zf. vnlpina and others). In the 

 forms with a foliose tendency the lobes are more uniform in size (E. 

 furfitracea) . The thallus never has that smooth shining appearance 

 so common in Cetraria \ the outer surface appears as though covered 

 with a very fine powder, due to the presence of a considerable de- 



