32S THALLOPHYTES. 



while the sorcdia are still included in the mother-thallus ; soredial branches, as they are 

 termed, are thus produced. 



We may now turn lo the consideration of the other elemental form out of which, in 

 addition to the Fungus-hyphae, the thallus of Lichens is constructed, the Gonidia. It has 

 already been suggested that these are nothing but Algae which are attacked and sur- 

 rounded in their growth by Ascomycetes, and serve as hosts to them, the capability of 

 assimilating inorganic materials being wanting on the part of their parasites. 



Passing over the views of the older lichenologists, which will be found collated in the 

 writings, cited below, of Baranetzky and of Schwendener, it may be pointed out here that 

 De Bary (Handbuch der physiol. Bot. vol. ii. p. 291) arrived at the following alternative 

 conclusions with respect to the gelatinous Lichens, such as Ephebe and similar forms ; 'that 

 either these Lichens are the completely developed fructifying states of plants the 

 incompletely developed forms of which have hitherto been placed among Algae, as 

 Nostocaceae and Chroococcaceae, or the Nostocaceae and Chroococcaceae are typical Algae 

 which assume the form of Collemae and Ephebae &c., in consequence of the penetration 

 into them of certain parasitic Ascomycetes the mycelium of which extends throughout the 

 growing thallus and often becomes attached to the cells filled with phycochrome 

 {Plectospora, Omphalaria). In the latter case the plants in question might be termed 

 Pseudolichens.' From the close of this quotation it appears that the writer does not 

 apply the latter alternative to the heteromerous Lichens at any rate. Soon afterwards 

 Famintzin and Baranetzky, and then the latter alone, published researches upon the 

 further changes which the gonidia of Lichens undergo when they are set free by the 

 decomposition of the hyphal tissue in water ^. Baranetzky comes to the conclusion that 

 * the gonidia of the heteromerous chlorophyll-containing Lichens {Physcia, E'vernia, Cla- 

 donia), as well as the heteromerous forms containing phycochrome {Peltigera) and of the 

 gelatinous Lichens (Co/lema), are capable of carrying on an entirely independent life out- 

 side the lichen-thallus. When set free, the lichen-gonidia appear to extend their cycle of 

 life; thus, for instance, the independently vegetating gonidia of Physcia, E'vernia, and Cla- 

 donia produce zoogonidia.' He also found that all the cells of the spherical n^asses com- 

 posed of the gonidia of Peltigera undergo a transformation so as to become extremely like 

 the interstitial cells of a Nostoc, and he did not doubt that this was their permanent 

 condition. 'Some, perhaps many,of the forms hitherto described asAlgae must be considered 

 as independently vegetating lichen-gonidia, for the present at any rate; such are Cystococcus, 

 Polycoccus and Nostoc.^ The researches of Schwendener carried on, in part earlier, in part 

 simultaneously and later, in the most careful manner, led to the opposite conclusion, that 

 the gonidia are in fact Algae which are more or less disturbed in their manner of life by 

 the Fungus which is parasitic upon them. He first definitely stated and explained this 

 view in his treatise 'Ueber die Algentypen der Flechtengonidien,' (Basel, 1869), as 

 applying to all Lichens. In this memorable work, which assigned to the Lichens for the 

 future their true systematic position among the Ascomycetes, he gives an account of those 

 genera of Algae which were to that time known as the hosts of lichen-fungi, that is, as 

 playing the part of gonidia. 



I. Bluish-green Algae. (Nostochineae.) 

 Name of group of Alga. Lichen in nuhich they occur as gcnidia, 



(i) Sirosiphoneae . . Ephebe, Spilonema, Polychidium. 



(2) Rivularieae . . Thamnidium, Lichina, Racoblenna. 



(3) Scylonemeae . . Heppia, Porccyphus. 



(4) Nostocaceae . . Collema,Lempholemma,Leptogium,Pannaria, Peltigera. 



(5) Chroococcaceae . . Omphalaria, Euchylium, Phylliscium. 



* Mem. de I'Acad. Imp. des Sci. de St. Petersbourg, 7th series, vol. XT. no. 9 and Melanges 

 biologiques tires du Bulletin de I'Acad. Imp. de St. Petersbourg, vol. VI. 1867. — [Ann. des Sci. Nat. 

 5th series, 1867, vol. VIII. pp. 137-144.] — Also Itzigssohn, Bot. Zeitg, 1868, [and Woronin, Ann. Sci, 

 Nat. XVI, 1872]. 



