124 MORPHOLOGY 



In the genus Cladonia, differentiation of the generative hyphae may 

 take place at a very early stage. Wainio"^ observed, in CI. caespiticia, a 

 trichogyne in a still solid podetium only 90 /a in height; usually they appear 

 later, and, where scyphi are formed, the carpogonium often arises at the 

 edge of the scyphus. Baur^ and WolfP* have furnished conclusive evidence 

 of the late appearance of the carpogonium in CI. pyxidata, CI. degenerans, 

 CI. furcata and CI. gracilis: in all of these species carpogonia with tricho- 

 gynes were observed on the edge of well-developed scyphi. Baur draws the 

 conclusion that the podetium is merely a vertical thallus, citing as additional 

 evidence that it also bears the spermogonia (or pycnidia), though at the 

 same time he allows that the apothecium may have played an important part 

 in its phylogenetic development. He agrees also with the account of the 

 first appearance of the podetium as described by Krabbe, who found that 

 it began with the hyphae of the gonidial zone branching upwards in a quite 

 normal manner, only that there were more of them, and that they finally 

 pierced the cortex. Krabbe also asserted that in the early stages the podetia 

 were without gonidia and that these arrived later from the open as colonists, 

 in this contradicting Wainio's statement that gonidia were carried up from 

 the primary thallus. 



It seems probable that the podetium — as Wainio and Baur both have 

 stated — is homologous with the apothecial stalk, though in most cases it is 

 completely transformed into a vertical thallus. If the view of their formation 

 from the gonidial zone is accepted, then they differ widely in origin from 

 normal branches in which the tissues of the main axis are repeated in the 

 secondary structures, whereas in this vertical thallus, hyphae from the 

 gonidial zone alone take part in the development. It must be admitted 

 that Baur's view of the podetium as essentially thalline seems to be strength- 

 ened by the formation of podetia at the centre of the scyphus, as in CI. 

 verticillata, which are new structures and are not an elongation of the 

 original conceptacular tissue. It can however equally be argued that the 

 acquired thalline character is complete and, therefore, includes the possibility 

 of giving rise to new podetia. 



The relegation of the carpogonium to a position far removed from the 

 base or primordium of the apothecium need not necessarily interfere with 

 the conception of the primordial tissue as homologous with the conceptacle; 

 but more research is needed, as Baur dealt only with one species, CI. pyxidata, 

 and Gertrude Wolff confined her attention to the carpogonial stages at the 

 edge of the scyphus. 



The Cladoniae require light, and inhabit by preference open moorlands, 

 naked clay walls, borders of ditches, exposed sand-dunes, etc. Those with 

 large and persistent squamules can live in arid situations, probably because 



1 Wainio 1897. ^ g^ur 1904. 3 Wolff 1905. 



