LICHEN GONIDIA 27 



F. Composite Nature of Thallus 



Schwendener' meanwhile was engaged on his study of lichen anatomy. 

 Though at first he adhered to the then accepted view of the genetic con- 

 nection between hyphae and gonidia, his continued examination of the 

 vegetative development led him to publish a short paper'' in which he 

 announced his opinion that the various blue-green and green gonidia were 

 really algae and that the complete lichen in all cases represented a fungus 

 living parasitically on an alga: in Ephebe, for example, the alga was a form 

 of Stigonema, in the CoUemaceae it was a species of Nostoc. In those lichens 

 enclosing bright green cells, the gonidia were identical with Cystococcus 

 humicola, while in Graphideae the brightly coloured filamentous cells were 

 those of Chroolepus {Trentepohlia). This statement he repeated in an 

 appendix to the larger work on lichens* and again in the following year* 

 when he described more fully the different gonidial algae and the changes 

 produced in their structure and habit by the action of the parasite: "though 

 eventually the alga is destroyed," he writes, "it is at first excited to more 

 vigorous growth by contact with the fungus, and in the course of generations 

 may become changed beyond recognition both in size and form." In support 

 of his theory of the composite constitution of the thallus, Schwendener 

 pointed out the wide distribution and frequent occurrence in nature of the 

 algae that become transformed to lichen gonidia. He claimed as further 

 proof of the presence of two distinct organisms that, while the colourless 

 filaments react in the same way as fungi on the application of iodine, the 

 gonidia take the stain of algal membranes. 



G. Synthetic Cultures 



Schwendener's "dual hypothesis," as it was termed, excited great interest 

 and no little controversy, the reasons for and against being debated with 

 considerable heat. Rees^ was the first who attempted to put the matter to 

 the proof by making synthetic cultures. For this purpose he took spores 

 from the apothecium of a Collema and sowed them on pure cultures oi Nostoc, 

 and as a result obtained the formation of a lichen thallus, though he did not 

 succeed in producing any fructification. He observed further that the 

 hyphal filaments from the germinating spore died off when no Nostoc was 

 forthcoming. 



Bornet* followed with his record of successful cultures. He selected for 

 experiment the spores of Physcia {Xanthoria) parietina and was able to 

 show that hyphae produced from the germinating spore adhered to the free- 



1 Schwendener i860, etc. '^ Schwendener 1867. ^ SchAvendener 1868, p. 195. 



■■ Schwendener 1869. ^ Kees 1871. * Bornet 187?. 



