36 



POPULAR HISTORY OF LICHENS. 



tribution is the opposite of that of crustaceous species, at- 

 taining their maximum development at low altitudes and 

 in tropical countries. Their habitat is most frequently the 

 bark of trees or the ground. Some species adhere to bases 

 of support by numerous minute fibrils or bundles of fila- 

 ments, proceeding from the under surface of the thallus; 

 these are denominated rhizina or fixurce. They resemble 

 rootlets in appearance, but not in function, acting chiefly, if 

 not solely, as means of adhesion. In some species growing 

 on moss they are long, pale-coloured, and delicate, as in 

 Peltigera ; in others, inhabiting the bark of trees, they are 

 very short, filiform, and black. In some foliaceous Lichens 

 there is a single and frequently central point of adhesion, 

 by means of a kind of disc or sucker. The crustaceous and 

 foliaceous thallus exhibit many intergradations and combi- 

 nations, which give rise to an infinity of irregular forms. 



The chief subdivisions of the vertical, or typically free, 

 thallus, are the fruticulose and filamentous. The former 

 consists of a shrub-like mass or aggregation of rigid, erect, 

 narrow, simple or branched, stem-like segments, which 

 sometimes arise from a common disc-like base, as in Spluz- 

 rojohoron, — sometimes spring separately from a small folia- 

 ceous or crustaceous horizontal thallus, as in some Claclonias 



