46 GUIDE TO THE STUDY OF LICHENS. 



which entered into the formation of the lichen or 

 lichen-group under consideration. The typical repro- 

 ductive organs of lichens are the soredia, which shall 

 receive consideration in another chapter. 



The apothecia have undergone certain changes dur- 

 ing the phylogenetic history of the lichen. These 

 changes are primarily for the furtherance of the func- 

 tion of assimilation, rather than that of spore-formation. 

 This is naturally to be expected since the spores are 

 of little value as reproductive organs. Many of the 

 higher lichens are constantly sterile. In some, the 

 original apothecial structure has been converted into 

 an assimilating organ, that is, a thallus. The vertical 

 thallus of the Gladonias, usually known as the pode- 

 tium, is doubtless such a modified apothecial structure. 

 Again, the " cup apothecia " of some lichens have be- 

 come greatly expanded, and contain within their in- 

 terior algae, thus acting as assimilating organs. In 

 others, particularly the lower forms, the apothecia re- 

 tain the structural characters of their fungal ancestors ; 

 that is, they do not bear algae, and hence do not take 

 part in the function of assimilation. Upon the pres- 

 ence or absence of algae are based the distinctions into 

 fungal and thalline apothecia. 



1. The Fungal Type. 



This apothecium is in all respects similar to the 

 apothecium of an Ascomycetous fungus. The algae of 

 the algal layer of the thallus do not enter into its 

 formation. It may be cup-shaped, as in the Discomy- 

 cetes, or immersed, as in the Pyrenomycetes. In the 



