92 GUIDE TO THE. STUDY OF LICHENS. 



not this be true. *S'. paschale is a typical rock-lichen. 

 It is said to be the first plant to develop upon the 

 volcanic lava of Vesuvius and Ischia. In various parts 

 of Europe wild animals feed upon it. In Lapland the 

 reindeer often feed upon it when Gladonia rangiferina 

 is scarce. 



Jf.. Cladonia. 



This genus is represented by a great number of 

 species which are so variable and diversified in form 

 that the systematist has considerable difficulty in de- 

 fining their limitations and describing them clearly. 

 It is, however, fortunate that nearly all of the repre- 

 sentatives are readily recognized as belonging to 

 Cladonia. A few species are also quite characteris- 

 tic as to form and once seen are not likely to be for- 

 gotten or mistaken for any other species ; such are (7. 

 rangiferina, G. papillaria, C. pyxidata, G. cariosa, 

 and a few others. 



Excepting in G. papillaria, the primary thallus is 

 foliose, although the individual thalli are minute ; 

 sometimes simple and entire, again considerably lobed 

 or even distinctly branching. Usually a large num- 

 ber of thalli occur together, more or less imbricate 

 with one side elevated. The upper surface is dis- 

 tinctly bright-green, the iower surface whitish. The 

 vertical thallus (secondary thallus, podetium) is 

 very variable in form ; from simple to much 

 branched. The simple forms, again, vary as to size 

 and structure ; some are nearly smooth, while others 

 bear numerous leafy outgrowths resembling the pri- 

 mary thalli ; some are cup-like toward the apex. 



