GROUP I. THALLOPHYTA : LICHENES. 



307 



(a) frulicose Lichen*, in which the thallus grows erect, branching in 

 a shrub-like manner. Of this form are the various species of Usnea 

 (Fig. 189 .4), and allied genera with a cylindrical thallus, which grow on 

 trees : Roccella tinctoria grows on rocks in regions bordering on the 

 Mediterranean ; from it and other allied Lichens litmus is prepared : 

 Ramalina and Evernia, with a ribbon-shaped flattened thallus, occur on 

 trees and wooden fences: Cetraria islandica is the Iceland Moss, which 

 forms a mucilaginous fluid when boiled with water : Cladonia, the Cup 

 Moss, has a scaly decumbent thallus, from which erect branches spring 

 bearing the apothecia ; Cladonia fimbriata is common ; Cladonia rangiferina, 

 the Reindeer Moss, occurs on moors. 



(i) foliaceous Lichens, in 

 which the thallus is flat- 

 tened and adheres to the 

 substratum: the green 

 (rarely bluish-green) algal 

 cells form a single layer 

 beneath the upper surface 

 (Fig. 188). The margin of 

 the thallus is usually lobed. 



Parmeiia (or Physcia) pa- 

 rietina occurs, with its 

 bright yellow thallus bear- 

 ing apothecia, on tree- 

 'trunks and walls, together 

 with other species of a grey 

 colour ; Sticta pulmonacea 

 (Fig. 189 B) has a reticu- 

 lated yellowish thallus, and 

 grows on tree-trunks : Pel- 

 tigera is represented by 

 several species which grow 

 on mossy banks in woods ; 

 the apothecia are borne on 

 the margin of the lobes of 

 the thallus. 



(c) crustaceous Lichens, in 

 which the thallus is usually 

 indefinite in outline, and 

 can often be scarcely dis- 

 tinguished from the substratum, the fructification alone being con- 

 spicuous. 



The Lichens of this form are extremely numerous. Among them may 

 be mentioned the Lecanorese. of which Lecanora subfusca occurs on the 

 trunks of trees : the Lecideaceae, which occur mainly on earth and rocks, 

 Lecidea geographica forming bright yellow incrustations of considerable 

 extent on silicious rocks : the Graphideae, of which Graphis scripta is 

 common on the trunks of Beeches and other trees. 



FIG. 188. Transverse section of the heteromerous 

 thallus of Sticta fuliginnsa (x 500). o Cortex of the 

 npper surface ; tt under surface ; m network of 

 hyphse forming the medullary layer ; g algal cells ; 

 r root-like outgrowths (rhizines) of the under sur- 

 face. (After Sachs.) 



