i 4 7 



leptts umbrind) are enclosed by a network of hyphae. In the tropical 

 representatives of this genus the thallus becomes greatly thickened 

 but never reaches beyond the crustaceous stage. 



The apothecia are markedly linear, variously curved and branched, 

 extending somewhat above the substratum and thallus. The 

 hypothecium (perithecium) and epithecium are dark colored in the 

 majority of species ; but in many of the southern forms the epithecium 

 and thallus are white. The paraphyses are somewhat longer than in 

 Opegrapha and quite granular. 



The spores are comparatively large, six to twelve-celled, color- 

 less, somewhat curved ; the exosporium is frequently wavy in out- 

 line. In G, elcgans spores are quite constantly wanting ; it is per- 

 haps a sterile form of G. scripta. 



The species are distinctly southern in their range, only a few oc- 

 curring in northeastern America. They occur upon the bark of 

 trees, preferably on smoother barks. 



PLATE 29. 

 GRAPHIS SCRIPTA Ach. 



1. Natural size. 



2. Portion magnified; a, apothecium. 



3. Section of apothecium : b and c, colorless and colored portions of 

 hypothecium. 



4. Section of thallus. 



5. Paraphyses and spore-sac. 



6. Spores. 



4. XYLOGRAPHA Fr. Syst. Mycol. 2: 197. 1823. 



This genus is represented by only a few species. It should per- 

 haps be placed before Graphis, as it is evidently lower in the scale 

 of development than that genus, but as far as the representatives of 

 these genera occur in the territory Xylographa reaches the greater 

 perfection. Its present systematic position will no doubt be changed 

 when the plants are more critically studied. 



The thallus begins its development below the substratum but 

 finally spreads over the surface, forming a thin, white layer. The 

 algae {Cystococcus humicola) occur in clusters enclosed by the hyphal 

 network. The hyphae intermingled with the remnants of dead algae 

 and even fragments of the substratum form a protective layer above 

 the algae. The rhizoidal hyphae are numerous and extend far into- 

 the wood on which they grow. 



