RADIATE THALLUS 



105 



In Letharia (L. vulpina, etc.) the structure is midway between Ramalina 

 and Usnea : the central axis is either a solid strand of chondroid hyphae or 

 several separate strands. 



Fig. 63 B. Usnea longissima Ach. A, transverse section of fibril x 85. B, a, chondroid axis; 

 b, gonidial tissue; c, cortex x 525 (after Schulte). 



In three other genera with upright fruticose thalli, SphaeropJiorus.Ar- 

 gopsis and Stereocaulon, rigidity is maintained by a medulla approaching the 

 chondroid type. In Sphaerophorus the species may have either flattened or 

 cylindrical branching stalks, but in all of them, the centre is occupied by 

 longitudinal strands of hyphae. Argopsis, a monotypic genus from Ker- 

 guelen, has a cylindrical branching thallus with a strong solid axis; it is 

 closely allied to Stereocaulon, a genus of familiar moorland lichens. The 

 central tissue of the stalks in Stereocaulon is also composed of elongate, 

 thick-walled conglutinate hyphae, formed into a strand which is, however, 

 not entirely solid. 



C. SURVEY OF MECHANICAL TISSUES 



Mechanical tissues scarcely appear among fungi, except perhaps as 

 stoutish cartilaginous hyphae in the stalks of some Agarics (Collybiae, etc.), 

 or as a ring of more compact consistency round the central hyphae of 

 rhizomorphic strands. It is practically a new adaptation of hyphal structure 

 confined to lichens of the fruticose group, where there is the same require- 

 ment as in the higher plants for rigidity, flexure and tenacity. 



Rigidity is attained as in other plants by groups or strands of mechanical 

 tissue situated close to the periphery, as they are so arranged in Rama- 

 lina and Cladonia; or the same end is achieved by a strongly developed 



