THE THALLUS 287 



Peltigera and Ne.phroma are also closely related in the character of the 

 fructification. It is a flat non-marginate disc borne on the edge of the 

 thallus : in Peltigera on the upper surface, in Nephroma on the under surface. 

 The remaining genus Solorina contains normally a layer of bright-green 

 algae, but, along with these, there are always present more or fewer Nosloc 

 cells, either in a thin layer as in S. crocea or as cephalodia in others, while, 

 in three species the algae are altogether blue-green. 



The members of the Peltigeraceae have a thick upper cortex of plecten- 

 chyma and in some cases strengthening veins, and long rhizinae on the 

 lower side. Some of the species attain a large size, and, in some, soredia 

 are formed, ah evidence of advance, this being a peculiarly lichenoid form 

 of reproduction. 



The Stictaceae form a parallel but more highly organized family, which 

 also includes closely related bright-green and blue-green series. They are 

 all dorsiventral, but they are mostly attached by a single hold-fast and the 

 lobes in some species suggest the fruticose type in their long narrow form. 

 A wide cortex of plectenchyma protects both the upper and the lower 

 surface and a felt of hairs replaces the rhizinae of other foliose lichens. In 

 the genus Sticta (including the section Stictind) special aeration organs, 

 cyphellae or pseudocyphellae, are provided ; in Lobaria these are replaced by 

 naked areas which serve the same purpose. 



Nylander 1 regarded the Stictaceae as the most highly developed of all 

 lichens, and they easily take a high place among dorsiventral forms, but it 

 is generally conceded that the fruticose type is the more highly organized. 

 In any case they are the highest reach of the phylum or phyla that started 

 with Pyrenopsidaceae and Collemaceae ; the lowly gelatinous thalli changing 

 to more elaborate structures with the abandonment of the gelatinous algal 

 sheath, as in the Pannariaceae, and with the replacement of blue-green by 

 bright-green gonidia. Reinke 2 , considers the Stictaceae as evolved from the 

 Pannariaceae more directly from the genus Massalongia. Their relationship 

 is certainly with Pannariaceae and Peltigeraceae rather than with Par- 

 meliaceae ; these latter, as we shall see, belong to a wholly different series. 



D. EVOLUTION OF ARCHILICHENS 



The study of Archilichens as of Phycolichens is complicated by the 

 many different kinds of fungi and algae that have entered into combination ; 

 but the two principal types of algae are the single-celled Protococcus group 

 and the filamentous Trentepohlia : as before only the broad lines of thalline 

 development will be traced. 



The elementary forms in the different series are of the simplest type a 

 somewhat fortuitous association of alga and fungus, which in time bears the 



1 See p. 126. 2 Reinke 1895. 



