I.K.HKNOLOGY OF ICELAND 147 



primary scales are fully formed, and in the centre, still older scales 

 with podetia, which are frequently placed distinctly in a circle 

 ("fairy ring"). The hypothallus can wander in the ground for years, 

 exactly as a fungal mycelium wanders; the podetia, on the other 

 hand, live a few years only, and are gradually replaced by new 

 ones. They are erect as long as they are alive, and end by dying 

 away at the base, so that, ultimately, they rot and fall down, as 

 they very rarely cohere with one another by haptera. 



This type is the most primitive of the fruticose lichens, as is 

 shown by the fact that it is still the vegetative thallus which keeps 

 on living, while the podetia, the curiously transformed apothecia- 

 stalks, die away and are destitute of all the peculiar contrivances 

 which are found in the type of podetium-wanderers, such as pro- 

 strate, creeping podetia, haptera, etc. 



It is, however, a type which is well-adapted to live on the 

 ground, which the hypothallus can easily penetrate. On the other 

 hand they are ill-adapted for life on the bark of trees or on stones, 

 which can only with great difficulty be penetrated by the hypo- 

 thallus. Nor do there occur, as far as is known, any species among 

 the bark or rock lichens which may be included among the hypo- 

 thallus-wanderers. On the other hand, they are more unfavourably 

 situated in regard to competition than are the podetium-wanderers, 

 which die away below and keep on growing at the apex, by which 

 means they can outgrow several other species. They are also far 

 rarer as regards individuals than are the excellently equipped po- 

 detium-wanderers, Alectoria ochroleuca, or Cladonia rangiferina. 



Cladonia rangiferina is the most highly developed of all the 

 podetium-wanderers we know. When the spore germinates, a 

 crust-shaped thallus is formed, but this is very rarely obtained, and 

 has been observed only by a few lichenologists, (Krabbe, Wainio 

 and Gallee), as it is very small and disappears very quickly. Upon 

 it the first podetia are developed, and they branch rapidly. The 

 primary thallus dies away and from henceforward the podetium is 

 left to look after itself; it gradually dies away at the base, but keeps 

 on growing "per secula" (Wainio) at the apex, so that it gradually 

 comes to rest upon a cake of lichen-peat made by itself. By the 

 dying aw r ay of the podetium, its lateral branches become gradually 

 isolated from one another; those that are placed somewhat hori- 

 zontally come gradually into touch with the surface of the ground, 

 and take hold of it at their apices by sending pencil-shaped haptera 



