THE THALLUS 283 



might become fertile. The granules with blue-green gonidia were stronger, 

 more healthy and capable of displacing those with Palmella, but not of 

 bearing apothecia, though spermogonia were embedded in them a first step, 

 according to Forssell, towards the formation of apothecia. These granules, 

 not having reached a fruiting stage, were reckoned to be of a more recent 

 type than those associated with Palmella. In other instances, however, the 

 line of evolution has been undoubtedly from blue-green to more highly 

 evolved bright-green thalli. 



The striking case of similarity between Psoroma hypnorum (bright-green) 

 and Pannaria rubiginosa (blue-green) may also be adduced. Forssell con- 

 siders that Psoroma is the more ancient form, but as the fungus is adapted 

 to associate with either kind of alga, the type of squamules forming the 

 thallus may be gradually transformed by the substitution of blue-green for 

 the earlier bright-green the Pannaria superseding the Psoroma. There is 

 a close resemblance in the fructification that is of the fungus in these two 

 different lichens. 



Hue 1 shares Forssell's opinion as to the greater antiquity of the bright- 

 green gonidia and cites the case of Solorina crocea. In that lichen there is 

 a layer of bright-green gonidia in the usual dorsiventral position, below the 

 upper cortex. Below this zone there is a second formed entirely of blue- 

 green cells. Hue proved by his study of development in Solorina that the 

 bright-green were the normal gonidia of the thallus, and were the only ones 

 present in the growing peripheral areas; the blue-green were a later addition, 

 and appeared first in small groups at some distance from the edge of the 

 lobes. 



The whole subject of cephalodia-development 2 has a bearing on this 

 question. These bodies always contain blue-green algae, and are always 

 associated with Archilichens. Mostly they occur as excrescences, as in 

 Stereocaulon and in Peltigera. The fungus of the host-lichen though normally 

 adapted to bright-green algae has the added capacity of forming later a sym- 

 biosis with the blue-green. This tendency generally pervades a whole genus 

 or family, the members of which, as in Peltigeraceae, are too closely related 

 to allow as a rule of separate classification even when the algae are totally 

 distinct. 



C. EVOLUTION OF PHYCOLICHENS 



The association of lichen-forming fungi with blue-green algae may have 

 taken place later in time, or may have been less successful than with the 

 bright-green: they are fewer in number, and the blue-green type of thallus 

 is less highly evolved, though examples of very considerable development 

 are to be found in such genera as Peltigera, Sticta or Nephrominm. 

 1 Hue 191 1 1 . 2 See p. 133. 



