RADIATE THALLUS 99 



those of Roccella have been found measuring 30 cm. in length ; those of 

 Ramalina reticulata, the largest of all the American lichens, extend to con- 

 siderably more. 



Lichens of filamentous growth are more or less cylindrical (Fig. 58). 

 They are in some species upright and of moderate length but in a few 



Fig. 58. Usnea barbata Web. (S. H., Photo.). 



pendulous forms they grow to a great length : specimens of Usnea longissima 

 have been recorded that measured 6 to 8 metres from base to tip. 



The radiate type of thallus occurs in most of the lichen groups but most 

 frequently in the Gymnocarpeae. In gelatinous Discolichens it is repre- 

 sented in the Lichinaceae. It is rare among Pyrenocarpeae : there is one 

 very minute British lichen in that series, Pyrenidium aciinellum, and one 

 from N. America, Pyrenothamnia, that are of fruticose habit. 



2. INTERMEDIATE TYPES OF THALLUS 



Between the foliose and the fruticose types, there are intermediate forms 

 that might be, and often are, classified now in one group and now in the 

 other. These are chiefly : Physcia {Anaptychia) ciliaris, Ph. leucomelas and 

 the species of Evernia. 



7—2 



