STRATOSE-RADIATE THALLUS 119 



follows on cessation of growth, or on perforation at the summit of the 

 podetium. Round this quiescent portion there rises a circle of minute 

 prominences which carry on the apical development. As they increase in 

 size, the spaces between them are bridged over by lateral growth, and the 

 scyphus thus formed is large or small according to the number of these 

 outgrowths. Apothecia or spermogonia may be produced at their tips, or 

 the vegetative development may continue. Scyphi formed in this manner 

 are also open or "pervious." 



d. GONIDIA OF THE SCYPHUS. Gonidia are absent in the early stages 

 of scyphus formation when it arises from degeneration of the apical 

 tissues, either fertile or vegetative ; but gradually they migrate from the 

 podetium, from the base of young outgrowths, or by furrows at the edge, and 

 so spread over the surface of the cup. Soredia may possibly alight, as 

 Krabbe insists that they do, and may aid in colonizing the naked area. 

 Their presence, however, would only be accidental ; they are not essential, 

 and scyphi are formed in many non-sorediate species such as Cl. verticillata. 

 The cortex of the scyphus becomes in the end continuous with that of the 

 podetium and is always similar in type. 



e. SPECIES WITHOUT SCYPHI. In species where the whole summit of 

 the podetium is occupied by an apothecium, as in Cl. bellidiflora, no scyphus 

 is formed. There is also an absence of scyphi in podetia that taper to a 

 point. In those podetia the hyphae are parallel to the long axis and remain 

 in connection with the external gonidial layer so that they are unaffected 

 by the central cavity. Instances of tapering growth are also to be found 

 in species that are normally scyphiferous such as Cl. fi mbriata subsp. fibula, 

 and Cl. cornuta, as well as in species like Cl. rangiferina that are constantly 

 ascyphous. 



The scyphus is considered by Wainio 1 to represent an advanced stage 

 of development in the species or in the individual, and any conditions that 

 act unfavourably on growth, such as excessive dryness, would also hinder 

 the formation of this peculiar lichen structure. 



D. BRANCHING OF THE PODETIUM 



Though branching is a constant feature in many species, regular dicho- 

 tomy is rare; more often there is an irregular form of polytomy in which one 

 of the members grows more vigorously than the others and branches again, 

 so that a kind of sympodium arises, as in Cl. rangiferina, Cl. sylvatica, etc. 



Adventitious branches may also arise from the podetium, owing to some 

 disturbance of the normal growth, some undue exposure to wind or to too 



1 Wainio 1897. 



