120 



spores, has been partially (Cladonia) or wholly (Thamnolia) sup- 

 planted by a secondary function, that of assimilation. 



The podetium, therefore, differs from the thalloid exciple, which 

 is likewise a secondary adaptation for the purposes of assimilation, 

 and is thus a thallus phylogenetically derived from an apothecial 

 stalk (stipe). The thalloid exciple is likewise a thallus derived 

 from the apothecium itself. 



The algal symbiont of the Cladoniaceae is Cystococcus humicola 

 Nag. (see Baeomvccs). The exact fungal ancestors are unknown ; 

 but they are probably derived from the Patellariaceae (see Plate 5 ) . 

 The different genera are mainly derived from different fungal groups 

 as is evident from the spore-characters. It is, however, very prob- 

 able that Thamnolia and Cladonia are derived from the same fun- 

 gal ancestor. 



This family also illustrates very beautifully the degenerative ten- 

 dency of apothecia and spores. Baeomyces^ the lowest genus, is 

 quite universally spore-bearing, likewise, Pilophoron ; Stereocau- 

 lon, which is the third genus in the series, presents occasional sterile 

 forms ; the higher Cladonias are quite uniformly sterile, as for ex- 

 ample C. rangifcrina, C. uncialis and others, and Thamnolia never 

 bears apothecia. 



The species of this family are quite common and widely dis- 

 tributed ; the soil is their more usual habitat though they also 

 occur upon tree-trunks, old fences, and rocks. 



KEY TO THE GENERA. 



Thallus (primary) crustaceous or warty. 



Apothecia borne on thick stipes or nearly sessile, i. Baeomyces. 

 Apothecia borne on podetia (secondary thalli). 



Podetia hollow ; spores simple. 2. Pilophoron. 



Podetia solid ; spores four-celled. 3. Stereocaulon. 

 Thallus (primary) foliose or wanting ; podetia hollow. 



Apothecia usually present ; spores simple. 4. Cladonia. 



Apothecia always wanting : podetia simple, or sparingly branched, 



pointed. 5- Thamnolia. 



i. BAEOMYCES Pers. Ust. Ann. 7: 19. I794- 1 

 The relationship of Baeomyces to the Cladoniaceae is based 

 principally upon its apothecial characters. The generic characters 



'According to O. Kuntze, the generic name Tubcrcularia Wigg., 1780, should be 

 used instead of this. 



