24 CONSTITUENTS OF THE LICHEN THALLUS 



hazard any theory as to the origin of gonidia, but he had observed spermatia 

 growing, much as did the cells of Oscillaria: by increase in length, and, by 

 subsequent branching, filaments were formed which surrounded the green 

 cells; these latter had meanwhile multiplied by repeated division till finally 

 a complete thallus was built up, the filamentous tissue being derived from 

 the spermatia, while the green layer came from the original gonidium. In 

 contrasting the development with that of Collema, he represents Nostoc as 

 a sterile product of a lichen and, like the gonidia of other licheins, only able 

 to form a lichen thallus when it encounters the fructifying spermatia. 



Braxton Hicks', a London doctor, some time later, made experiments 

 with Chroococcus algae- which grew in plenty on the bark of trees, and 

 followed their development into a lichen thallus. He further claimed to 

 have observed a Chlorococcus, which was associated with a Cladonia, divide 

 and form a Pabnella stage. 



C. Culture Experiments with the Lichen Thallus 



It had been repeatedly stated that the gonidia might become independent 

 of the thallus, but absolute proof was wanting until Speerschneider", who 

 had turned his attention to the subject, made direct culture experiments 

 and was able to follow the liberation of the green cells. He took a thinnish 

 section of the thallus of Hagenia {Pkyscia) ciliaris, Sind,hy keeping it moist, 

 he was able to observe that the gonidial cells increased by division; the 

 moist condition at the same time caused the colourless filaments to die 

 away. This method of investigation was to lead to further results. It was 

 resorted to by Famintzin and Baranetsky^ who made cultures of gonidia 

 extracted from three different lichens, Physcia {Xantkoria) parietina, 

 Evernia furfuracea and Cladonia sp. They were able to observe the growth 

 and division of the green cells and, in addition, the formation of zoospores. 

 They recognized the development as entirely identical with that of the 

 unicellular green alga, Cystococcus humicola Naeg. Baranetsky* continued 

 the experiments and made cultures of the blue-green gonidia of Peltigera 

 canina and of Collema pulposum. In both instances he succeeded in isolating 

 them from the thallus and in growing them in moist air as separate 

 organisms. He adds that "many forms reckoned as algae, may be con- 

 sidered as vegetating lichen gonidia such as Cystococcus, Pblycoccus, Nostoc, 

 etc." Meanwhile Itzigsohn= had further demonstrated by similar culture 

 experiments that the gonidia of Peltigera canina corresponded with the 

 algae known as Gloeocapsa monococca Kiitz., and as P oly coccus punctiformis 

 Kutz. 



' Hicks i860 and 1861. ^ Speerschneider 1853. » Famintzin and Baranetsky 1867. 



^ Baranetsky 1869. ° Itzigsohn 1867. 



