22 



CONSTITUENTS OF THE LICHEN THALLUS 



Wallroth's terminology and his views of the function of the gonidia were 

 accepted as the true explanation for many years, the opinion that they were 

 solely reproductive bodies being entirely in accordance with the well-known 

 part played by soredia in the propagation of lichens and soredia always 

 include one or more green cells. 



B. GONIDIA CONTRASTED WITH ALGAE 



In describing the gonidia of the Graphideae Wallroth 1 had pointed out 

 their affinity with the filaments of Chroolepus (Trentepohlid) umbrina. He 

 considered these and other green algae when growing loose on the trunks of 

 trees to be but "unfortunate brood-cells" which had become free and, though 

 capable of growth and increase, were unable to form again a lichen plant. 



Further observations on gonidia were made by E. Fries 2 : he found that 

 the green cells escaped from the lichen matrix and produced new individuals; 

 and also that the whole thallus in moist localities might become dissolved 

 into the alga known as Protococcus viridis; but, he continues, "though these 

 Protococcus cells multiplied exceedingly, they never could rise again to the 

 perfect lichen." Kiitzing 3 , in a later account of Protococcus viridis, also 

 recognized its affinity with lichens; he stated that he could testify from 

 observation that, according to the amount of moisture present, it would 

 develop, either in excessive moisture to a filamentous alga, or in drier con- 

 ditions "to lichens such as Lecanora subfusca or Xanthoria parietina." 



A British botanist, G. H. K. Thwaites 4 , at one time superintendent of 

 the botanical garden at Peradeniya in Ceylon, published a notable paper 



on lichen gonidia in which he pointed out 

 that as in Collema the green constituents 

 of the thallus resembled the chains of Nostoc, 

 so in the non-gelatinous lichens, the green 

 globose cells were comparable or identical with 

 Pleurococcus, and Thwaites further observed that 

 they increased by division within the lichen 

 thallus. He insisted too that in no instance were 

 gonidia reproductive organs: they were essen- 

 tial component parts of the vegetative body and 

 necessary to the life of the plant. In a further 

 paper on Chroolepus cbeneus Ag., a plant con- 

 sisting of slender dark -coloured felted fila- 

 ments, he described these filaments as being 

 composed of a central strand which closely 

 resembled the alga Chroolepus, and of a surrounding sheath of dark-coloured 



Fig- 3- Cocnogonium ebcneum A. L. 

 Sm. Tipof lichen filament, the alga 

 overgrown by dark fungal hyphae 

 x 600. 



1 Wallroth 1825, 1, p. 303. 

 3 Kiitzing 1843. 



2 Fries 1831, pp. Ivi and Ivii. 



4 Thwaites 1849, pp. 219 and 241. 



