THE THALLUS 293 



are colourless and mostly simple, but there are also changes in form and 

 septation not commensurate with thalline advance, as has been already noted. 

 Thus in Gomphillus, with primitive thallus and podetium, the spores are 

 long and narrow with about 100 divisions. 



1. Origin of Cladonia. There is no difficulty in deriving Cladoniaceae 

 from Lecidea, or, more exactly, from some crustaceous species of the section 

 Biatora in which the apothecia — as in Cladoniaceae — are waxy and more 

 or less light-coloured and without a thalline margin. In only a very few 

 isolated instances has a thalline margin grown round the Cladonia fruit. 



There are ten genera included in the Cladoniaceae, of which five are 

 British. Considerable study has been devoted to the elucidation of develop- 

 mental problems within the family by various workers, more especially in the 

 large and varied genus Cladonia which is complicated by the presence of 

 the two thalli. The family is monophyletic in origin, though many subordi- 

 nate phyla appear later. 



2. Evolution of the Primary Thallus. At the base of the series we 

 find here also an elementary granular thallus which appears in some species 

 of most of the genera. In Gomphillus, a monospecific British genus, the 

 granules have coalesced into a continuous mucilaginous membrane. In 

 Baeomyces, though mostly crustaceous, there is an advance to the squamulose 

 type in B. placophyllus, and in two Brazilian species described by Wainio, 

 one of which, owing to the form of the fronds, has been placed in a separate 

 genus Heteromyces. The primary thallus becomes almost foliose also in 

 Gymnoderma coccocarpum from the Himalayas, with dorsiventral stratose 

 arrangement of the tissues, but without rhizinae. The greatest diversity 

 is however to be found in Cladonia where granular, squamulose and almost 

 foliose thalli occur. The various tissue formations have already been 

 described ^ 



3. Evolution of the Secondary Thallus. Most of the interest 

 centres round the development and function of the podetium. In several 

 genera the primordium is homologous with that of an apothecium; its 

 elongation to an apothecial stalk is associated with delayed fructification, 

 and though it has taken on the function of the vegetative thallus, the purpose 

 of elongation has doubtless been to secure good light conditions for the 

 fruit, and to facilitate a wide distribution of spores : therefore, not only in 

 development but in function, its chief importance though now assimilative 

 was originally reproductive. The vegetative development of the podetium is 

 correlated with the reduction of the primary thallus which in many species 

 bears little relation in size or persistence to the structure produced from it, 

 as, for instance, in Cladonia rangiferina where the ground thallus is of the 



1 See Chap. III. 



