THE THALLUS 295 



of the scyphus and branching of the podetium, with (3) differences in colour 

 both in the vegetative thallus and in the apothecia. 



Wainio has indicated the course of evolution on the following lines : 

 (i) the crustaceous thallus is monophyletic in origin and here as elsewhere 

 precedes the squamulose. The latter he considers to be also monophyletic, 

 though at more than one point the more advanced and larger foliose forms 

 have appeared : (2) the primitive podetium was subulate and unbranched, 

 and the apex was occupied by the apothecium. Both scyphus and branching 

 are later developments indicating progress. They are in both cases associated 

 with fruit-formation scyphi generally arising from abortive apothecia 1 , 

 branching from aggregate apothecia. In forms such as Cl. fimbriata, where 

 both scyphiferous and subulate sterile podetia are frequent, the latter (sub- 

 species fibula) are retrogressive, and reproduce the ancestral pointed pode- 

 tium. (3) In subgen. Cenomyce, with a squamulose primary thallus, there is 

 a sharp division into two main phyla characterized by the colour of the 

 apothecia, brown in Ochrophaeae the colour being due to a pigment and 

 red in Cocciferae where the colouring substance is a lichen-acid, rhodocladonic 

 acid. In the brown-fruited Ochrophaeae there are again several secondary 

 phyla. Two of these are distinguished primarily by the character of the 

 branching : (a) the Chasmariae in which two or several branches arise from 

 the same level, entailing perforation of the axils (Cl. fttrcata, Cl. rangi- 

 formis, Cl. squamosa, etc.), the scyphi also are perforated. They are further 

 characterized by peltate aggregate apothecia, this grouping of the apothecia 

 according to Wainio being the primary cause of the complex branching, 

 the several fruit stalks growing out as branches. The second group (), the 

 Clausae, are not perforated and the apothecia are simple and broad-based 

 on the edge of the scyphus (Cl. pyxidata, Cl. fimbriata, etc.), or on the tips 

 of the podetia (Cl. cariosa, Cl. leptophylla, etc.). A third very small group 

 also of Clansae called (c) Foliosae has very large primary squamules and 

 reduced podetia (Cl. foliacea, etc.), while finally (d) the Ochroleucae, none of 

 which is British, have poorly developed squamules and variously formed 

 yellowish podetia with pale-coloured apothecia. 



The Cocciferae represent a phylum parallel in development with the 

 Ochrophaeae. The species have perhaps most affinity with the Clausae, the 

 vegetative thallus both the squamules and the podetia being very much 

 alike in several species. Wainio distinguishes two groups based on a differ- 

 ence of colour in the squamules, glaucous green in one case, yellowish in 

 the other. 



6. CAUSES OF VARIATION. External causes of variation in Cladonia are 

 chiefly humidity and light, excess or lack of either effecting changes which 

 may have become fixed and hereditary. Minor changes directly traceable 



1 See Chap. III. 



