i68 



thallus is very marked (ex. P. centrifuga)^ in others (P. pcrlata, 

 P. rudecta, P. latissiina) the lobes are very large and the dichoto- 

 mous branching is not distinct, even approaching the entire form. 

 Branching of the thallus is, however, characteristic of the genus, in 

 this respect approaching the characters of the thallus in Phvscia^ but in 

 general it is larger and shows higher specializations. The prevailing 

 colors of the upper surface are greenish grey and greenish brown ; 

 the lower surface is brown to black, usually bearing black rhizoids ; 

 cilia sometimes occur along the margin. Soredia are also numerous 

 on the upper surface of the thallus as well as along the margin. 

 Sometimes numerous isidioid branches form nearest the middle of 

 old thalli (especially in P. Borreri}, finally nothing remaining but a 

 broken-down thallus bearing them in large numbers ; there is no 

 doubt that these outgrowths serve as vegetative propagative organs, 

 similar to the soredia. 



Both upper and lower cortical layers are present and are far more 

 characteristically cortical than in any of the genera heretofore de- 

 scribed. The cells are considerably shortened, with firm walls and 

 rather large lumina. The algal layer is clearly defined. The 

 medullary layer consists of a loose network of hyphae especially 

 adapted to act as an aerating tissue. In some species (P. physodes} 

 there is a highly specialized aerating tissue, consisting of an air- 

 space just above the lower cortical layer. 



The apothecia vary from medium to very large ; they are sessile 

 upon the upper surface of the thallus ; in general it may be stated 

 that they are comparatively rare, especially in those forms with 

 numerous soredia. The enormous development of the thalloid ex- 

 ciple would make it seem probable that the prime significance of 

 this structure is to aid in assimilation rather than in reproduction by 

 spore formation ; each apothecium is in reality a secondary thal- 

 lus having a combined radial and centric structure ; the epithe- 

 cium is usually brown in color; the paraphyses are simple, very 

 rarely branching, rather rigid with slight coloration at the upper 

 ends ; the hypothecium is colorless and consists of two, usually more 

 or less distinct layers, much as in S-peer Schneider a. 



The spores are simple, colorless, elliptical and quite constant in 

 size and form ; they very frequently contain two oil-globules which 

 to casual observance presents the appearance of a two-celled spore ; 

 the spore-wall is perfectly colorless, quite thin and somewhat 

 gelatinous. 



