STRATOSE-RADIATE THALLUS 113 



between the older cortical filaments and so provide for the expansion as 

 well as for the renewal of the cortical tissue. 



b. GONIDIAL TISSUE. The gonidia consisting of Protococcaceous algae 

 form a layer immediately below the cortex. Isolated green cells are not 

 unfrequently carried up by the growing hyphae into the cortical region, but 

 they do not long survive in this compact non-aerated tissue. Their empty 

 membranes can however be picked out by the blue stain they take with 

 iodine and sulphuric acid. 



Krabbe 1 has described the phases of development in the growing region : 

 he finds that differentiation into pith, gonidial zone and cortex takes place 

 some little way back from the edge. At the extreme apex the hyphae lie 

 fairly parallel to each other; further back, they branch upwards to form the 

 cortex, and to separate the masses of multiplying gonidia, by pushing 

 between them and so spreading them through the whole apical tissue. The 

 gonidia immediately below the upper cortex, where they are well-lighted, 

 continue to increase and gradually form into the gonidial zone; those that 

 lie deeper among the medullary hyphae remain quiescent, and before long 

 disappear altogether. 



Where the squamules assume the upright position (as in Cladonia cervi- 

 cornis), there is a tendency for the gonidia to pass round to the lower 

 surface, and soredia are occasionally formed. 



c. MEDULLARY TISSUE. The hyphae of the medulla are described by 

 Wainio as having long cells with narrow lumen, and as being encrusted 

 with granulations that may coalesce into more or less detachable granules ; 

 in colour they are mostly white, but pale-yellow in Cl.foliacea and blood-red 

 in Cl. miniata, a subtropical species. They are connected at the base of the 

 squamules with a filamentous hypothallus which penetrates the substratum 

 and attaches the plant. In a few species rhizinae are formed, while in others 

 the hyphae of the podetium grow downwards, towards and into the sub- 

 stratum as a short stout rhizoid. 



d. SOREDIA. Though frequent on the podetia, soredia are rare on the 

 squamules, and, according to Wainio 2 , always originate at the growing 

 region, from which they spread over the under surface rather sparsely in 

 Cl. cariosa, Cl. squamosa, etc., but abundantly in Cl. digitata and a few others. 

 In some instances, they develop further into small corticate areolae on the 

 under surface (Cl. coccifera, Cl. pyxidata and Cl. squamosa). 



1 Krabbe 1891. 2 Wainio 1897. 



S. L. 



