STRATOSE THALLUS 81 



B. TISSUES OF SQUAMULOSE THALLUS 



The anatomical structure of the squamules is in general somewhat 

 similar to that of the crustaceous thallus: an upper cortex, a gonidial zone, 

 and below that a medullary layer of loose hyphae with sometimes a lower 

 cortex. 



1. The upper cortex, as in crustaceous lichens, is generally of the 

 "decomposed" 1 or amorphous type: interlaced hyphae with thick gelatinous 

 walls. A more highly developed form is apparent in Parmeliella and 

 Pannaria where the upper cortex is formed of plectenchyma, while in the 

 squamules of Heppia the whole structure is built up of plectenchyma, with 

 the exception of a narrow band of loose hyphae in the central pith. 



2. The gonidia are Myxophyceae or Chlorophyceae; the squamules in 

 some instances may be homoiomerous as in Lepidocollema, but generally 

 they belong to the heteromerous series, with the gonidia in a circumscribed 

 zone, and either continuous or in groups. Friedrich 2 held that, as in crus- 

 taceous lichens the development of the gonidial as compared with the other 

 tissues depended on the substratum. The squamules of Pannaria micro- 

 phylla on sandstone were loo/i thick, and the gonidial layer occupied 80 or 

 90 yu, of the whole 3 . With that may be compared Placodium Garovagli on 

 lime-containing rock: the gonidial layer measured only 50 //, across, the 

 pith hyphae 280/4 and the rhizoidal hyphae that penetrated the rock 500 /*. 



3. The medullary layer, as a rule, is of closely compacted hyphae which 

 give solidity to the squamules; in those of Heppia it is almost entirely 

 formed of plectenchyma. 



4. The lower cortex is frequently little developed or absent, especially 

 when the squamules are closely applied to the support as in some species 

 of Dermatocarpon. In some of the squamulose Lecanorae (L. crassa and 

 Z. saxicola) the lowest hyphae are somewhat more closely interwoven; 

 they become brown in colour, and the lichen is attached to the substratum 

 by rhizoid-like branches. In Lecanora lentigera there is a layer of parallel 

 hyphae along the under surface. Further development is reached when 

 a plectenchyma of thick-walled cells is formed both above and below, as in 

 Psoroma hypnorum, though on the under surface the continuity is often 

 broken. The squamules of Cladoniae are described under the radiate-stratose 

 series. 



1 See p. 83. 2 Friedrich 1906. 3 See p. 76. 



S. L. 



