LICHEN GONIDIA 



23 



cells (Fig. 3): "occasionally," he writes, "the internal filament protrudes 

 beyond the investing sheath, and may then be seen to consist of oblong 

 cells containing the peculiar reddish oily-looking endochrome of Chroolepus!' 

 Thwaites placed this puzzling plant in a new genus, Cystocoleus, at the same 

 time pointing out its affinity with the lichen genus Coenogonium. The 

 plant is now known as Coenogonium ebeneum. Thwaites was on the 

 threshold of the discovery as to the true nature of the relationship between 

 the central filament and the investing sheath, but he failed to take the next 

 forward step. 



Very shortly after, Von Flotow^ published his views on some other 

 lichen gonidia. He had come to the conclusion that the various species of 

 the alga, Gloeocapsa, so frequently found in damp places, among mosses and 

 lichens, were merely growth stages of the gonidia of Ephebe pubescens, and 

 bore the same relation to Ephebe as did Lepra viridis {Protococcus) to Par- 

 melia. The gonidium of Ephebe is the gelatinous 

 filamentous blue-green alga Stigonema (Fig. 4), 

 and the separate cells are not unlike those of 

 Gloeocapsa. Flotow had also demonstrated that the 

 same type of gonidium was enclosed in the cepha- 

 lodia of Stereocaulon. Sachs^ too, gave evidence as 

 to the close connection between Nostoc and Col- 

 lema. He had observed numerous small clumps of 

 the alga growing in proximity to equally abundant 

 thalli of CoUema, with every stage of development 

 represented from one to the other. He found cases 

 where the gelatinous coils of Nostoc chains were 

 penetrated by fine colourless filaments "as if in- 

 vaded by a parasitic fungus." Later these threads were seen to be attached 

 to some cell of the Nostoc trichome. Sachs concluded, however, from very 

 careful examination at the time, that the colourless filaments were produced 

 by the green cells. As growth proceeded, the coloured Nostoc chains became 

 massed towards the upper surface, while the colourless filaments tended to 

 occupy the lower part of the thallus. He calculated that during the summer 

 season the metamorphosis from Nostoc to a fertile Collema thallus took from 

 three to four months. He judged that in favourable conditions the change 

 would inevitably take place, though if there should be too great moisture no 

 Collema would be formed. His study of Cladonia was less successful as he 

 mistook some colonies of Gloeocapsa for a growth condition of Cladonia 

 gonidia, an error corrected later by Itzigsohn^ 



But before this date Itzigsohn* had published a paper setting forth his 

 views on thallus formation, which marked a distinct advance. He did not 



' Flotow 1850. ^ Sachs 1855. ^ Itzigsohn 1855. * Itzigsohn 1854. 



Fig. 4. Ephebe pubescens '^y\. 

 Tip of lichen filament x 600. 



