294 PHYLOGENY 



scantiest and very soon disappears, while the podetial thallus continues to 

 grow indefinitely and to considerable size. 



4. COURSE OF PODETIAL DEVELOPMENT. In Baeomyces the podetial 

 primordium is wholly endogenous in some species, but in others the 

 outer cortical layer of the primary thallus as well as the gonidial hyphae 

 take part in the formation of the new structure which, in that case, is simply 

 a vertical extension of the primary granule. This type of podetium called 

 by Wainio'a pseudopodetium also recurs in Pilophorus and in Stereocanlon. 

 To emphasize the distinction of origin it has been proposed to classify these 

 two latter genera in a separate family, but in that case it would be necessary 

 to break up the genus Baeomyces. We may assume that the endogenous 

 origin of the "apothecial stalk" is the more primitive, as it occurs in the 

 most primitive lecideine lichens, whereas a vertical thallus is always an 

 advanced stage of vegetative development. 



Podetia are essentially secondary structures, and they are associated 

 both with crustaceous and squamulose primary thalli. If monophyletic in 

 origin their development must have taken place while the primary thallus 

 was still in the crustaceous stage, and the inherited tendency to form podetia 

 must then have persisted through the change to the squamulose type. In 

 species such as Cl. caespiticia the presence of rudimentary podetia along 

 with large squamules suggests a polyphyletic origin, but Wainio's 1 opinion is 

 that such instances may show retrogression from an advanced podetial form, 

 and that the evidence inclines to the monophyletic view of their origin. 



The hollow centre of the podetium arises in the course of development 

 and is common to nearly all advanced stages of growth. There are how- 

 ever some exceptions : in Glossodium aversum, a soil lichen from New 

 Granada, and the only representative of the genus, a simple or rarely forked 

 stalk about 2 cm. in height rises from a granular or minutely squamulose 

 thallus. The apothecium occupies one side of the flattened and somewhat 

 wider apex. There is no external cortex and the central tissue is of loose 

 hyphae. In Thysanothecium Hookeri, also a monotypic genus from Australia, 

 the podetia are about the same height, but, though round at the base, they 

 broaden upwards into a leaf-like expansion. The central tissue below is of 

 loose hyphae, but compact strands occur above, where the apothecium 

 spreads over the upper side. The under surface is sterile and is traversed 

 by nerve-like strands of hyphae. 



5. VARIATION IN CLADONIA. It is in this genus that most variation is to 

 be found. Characters of importance and persistence have arisen by which 

 secondary phyla may be traced within the genus: these are mainly (i) the 

 relative development of the horizontal and vertical structures, (2) formation 



1 Wainio 1897. 



