198 



Chromium are usually classed under a distinct family-group {Pel ti- 

 ger ei) ; they have, however, so many characters in common with 

 Pannaria and Physina on the one hand and Stictina and Sticta on 

 the other that it seems perfectly justifiable to include them in the 

 family ; it is also very difficult to decide upon what characters to 

 base further subdivisions. Upon careful consideration it becomes 

 evident that the characters of the apothecia, of the thallus, or of the 

 algae taken separately will not suffice to establish the family or fam- 

 lies. For instance, Pannaria, Psoroma, Hcppia and Physina are 

 closely related according to their spore-characters and the general 

 features of the thallus and apothecium, but differ widely as to the al- 

 gal symbionts. Lichina and Physma are closely related by their algae, 

 but differ widely as to the general conformations of the thallus. 

 Stictina and Sticta are unquestionably closely related, in fact they 

 are quite frequently combined in one genus, yet the algal symbionts 

 are constantly different ; from these varying characters it is evident 

 that careful comparative study is necessary to find the true relation- 

 ships of the genera belonging to the family ; it is also evident that 

 constant family characters are not to be found or are at least very 

 few; the algal characters are, perhaps, most reliable; with several 

 exceptions the symbiotic algae belong to the Cyanophyceae, charac- 

 terized by a blue-green color ; in the first five genera the thallus is 

 fruticose (excepting Lecothecium) ; in the remaining genera the 

 thallus is typically foliose and shows some high structural adapta- 

 tions ; the apothecial characters differ considerably and likewise show 

 structural adaptations indicating a higher specialization ; since these 

 specializations are quite variable they will be discussed in the generic 

 descriptions in so far as they have not already been described. 



The question may be raised why the fruticose types are considered 

 lower in this family and higher in the Parmeliaceae. The fruticose 

 types of the Pannariaceae are small and present a low organization 

 as lichens, while just the reverse is true of the fruticose types of the 

 Parmeliaceae. To solve the relationship of the fruticose to foliose 

 thalli and their relative position in the scale of development as lichens 

 it is necessary to consider the morphology of the algal as well as 

 fungal ancestral forms in so far as that is possible. As already in- 

 dicated, the morphology of the ancestral forms of the symbionts has a 

 marked influence upon the development of the lichen. This is well 

 illustrated in Ephcbe and Lichina \ both are fruticose lichens, though 



