120 MORPHOLOGY 



great light, or owing to some external injury. They originate ' from the 

 gonidial tissue in the same way as does the podetium from the primary 

 thallus; the parallel hyphae of the main axis take no part in their develop- 

 ment. 



In a number of species secondary podetia arise from the centre of the 

 scyphus constantly in Cl. verticillata and Cl, cervicornis, etc., accidentally 

 or rarely in Cl. foliacea, Cl. pyxidata, Cl. fimbriata, etc. Wainio 1 has stated 

 that they arise when the scyphus is already at an advanced stage of growth 

 and that they are to be regarded as adventitious branches. 



The proliferations from the borders of the scyphus are in a different 

 category. They represent the continuity of apical growth, as the edges of 

 the scyphus are but an enlarged apex. These marginal proliferations thus 

 correspond to polytomous branching. In many instances their advance is 

 soon stopped by the formation of an apothecium and they figure more as 

 fruit stalks than as podetial branches. 



E. PERFORATIONS AND RETICULATION OF THE PODETIUM 



Perforations in the podetial wall at the axils of the branches are constant 

 in certain species such as Cl. rangiferina, Cl. uncialis, etc. They are caused 

 by the tension of the branches as they emerge from the main stalk. 

 A tearing of the tissue may also arise in the base of the scyphus, due to its 

 increase in size, which causes the splitting of the diaphragm at the bottom 

 of the cup. 



Among the Cladoniae the reticulate condition recurs now and again. 

 In our native Cladonia cariosa the splitting of the podetial wall is a constant 

 character of the species, the carious condition being caused by unequal 

 growth which tears apart the longitudinal fibres that surround the central 

 hollow. 



A more advanced type of reticulation arises in the group of the Clathrinae 

 in which there is no inner chondroid cylinder. In Cladonia aggregata, in 

 which the perforations are somewhat irregular, two types of podetia have 

 been described by Lindsay 2 from Falkland Island specimens: those bearing 

 apothecia are short and broad, fastigiately branched upwards and with 

 reticulate perforations, while podetia bearing spermogonia are slender, elon- 

 gate and branched, with fewer reticulations. An imperfect network is also 

 characteristic of Cl. Sullivani, a Brazilian species. But the most marvellous 

 and regular form of reticulation occurs in Cl. retepora, an Australian lichen 

 (Fig. 71): towards the tips of the podetia the ellipsoid meshes are small, 

 but they gradually become larger towards the base. In this species the 

 outer tissue, though of parallel hyphae, is closely interwoven and forms 

 1 Wainio 1897. 2 Lindsay 1859, p. 171. 



