187 



more or less lobate. The color varies from light grey to greenish 

 grey. The algae (Cystococcus humicola) are normal in appearance 

 and do not seem to occur very plentifully. 



The apothecia are minute, globose and emersed in the warty 

 elevations where they occur in colonies of five or six. Usually there 

 is no perceptible pore-opening. In the higher forms the part of the 

 thallus above the disk becomes more or less sunken, which to casual 

 observance may give the appearance of an apothecial disk. In 

 the majority of species the apothecium remains globose, being almost 

 entirely enclosed by the usually colorless perithecium. 



The paraphyses are long, slender and more or less branched ; 

 they are quite gelatinous and perfectly colorless. The spores are 

 mostly very large, simple, colorless and usually one to three in each 

 enormously developed spore-sac. The spore-wall is very thick, and 

 in some species (jP. communis) the inner membrane is reticulately 

 thickened. During germination hyphae begin to form at the thin 

 areas. The plasmic contents are granular, sometimes bearing oil 

 globules. 



The Pertusarias are also characterized by the numerous soralia 

 which form upon the upper surface of the thallus, often in rounded 

 masses, almost white in color, which may on casual observation be 

 taken for apothecia ; these have been made the subject of special 

 study by Darbishire. 1 



Most of the representatives are cosmopolitan, occurring either 

 upon rock or bark, but some also occur on mosses. 



PLATES 57 and 58. 



Pertusaria communis DC. 



1. Natural size. 



2. Semidiagramatic section of apothecium. a, cortical layer; b, algal 

 layer; d, apical pore through which the spores escape; c, medullary 



tissue ; e, thecium. 



3. Section of apothecium. 



4. Section of thallus. 



5. Paraphyses and spore-sac. a, young spore-sac; b, later stage; c, 

 mature spore-sac with three spores; d, sjDore-sac with apical seg- 

 ment removed to allow the escape of the spores. 



6. Mature spore, a, exosporium ; b, endosporium with reticular thick- 

 enings as seen in optical section ; c, oil globules. 



1 Die deutschen Pertusariaceen mit besonderer Beriicksichtigung ihrer Soredien- 

 bildung. Engler's bot. Jahrb. 22 : 593-679. 1897. 



