207 



is occupied by the alga-bearing "tissue ; the alga in this case is a 

 species of Scytonema ; its branching, and relative position of the 

 cells are difficult to observe even in the most careful sectioning ; they 

 seem to have undergone considerable modification in structure since 

 their association as lichens, and for this reason it is also difficult to de- 

 termine the species. In the thallus the algal chains extend vertically 

 parallel with the hyphal cells ; at certain points they approach very 

 near the upper surface. The color of the thallus is brown above as 

 well as below, becoming quite dark with age. 



The apothecia are comparatively large, situated in depressions of 

 the thallus. The apothecium itself is disk-like, but not raised above 

 the general surface of the thallus ; there is, therefore, no noticeable 

 line of demarcation between the thallus and apothecium ; one passes 

 into the other without any structural differences observable by the nak- 

 ed eye. The hypothecium is colorless and cortical in structure. The 

 thecium is brown, much as in Psoroma. The paraphyses and spore- 

 sacs are rather long and colorless ; the spores are colorless, simple, 

 thin-walled with somewhat granular contents and quite variable in 

 size and form. 



H. Desfreuxii seems to be southern in its distribution, though it 

 extends well into the territory. It occurs upon the soil, somtimes 

 upon coarse sand. 



PLATE 70. 

 Heppia Despreuxii (Mont.) Tuck. 



1. Thallus natural size ; a, apothecia. 



2. Section of apothecium. 



3. Section of thallus. 



4. Paraphyses and spore-sac. 



5. Spores. 



6. Algae and hyphae. 



8. Pannaria Delis.; Kbr. Syst. Lich. Ger. 105. 1855. 



This is another much confused genus, the confusion primarily 

 caused by failure to distinguish or recognize the proper algae. 



The thallus is quite distinctly foliose in the majority of species, 

 though the lobules are usually small. In the lower species the 

 thallus presents a"crustaceous appearance, but upon careful examina- 

 tion it is found, however, that both upper and lower cortical layers 



