28 



CONSTITUENTS OF THE LICHEN THALLUS 



growing cells of Protococcus^ viridis and formed the early stages of a lichen 

 thallus. Woronin 2 contributed his observations on the gonidia of Parmelia 

 (Physcia) pulverulenta which he isolated from the thallus and cultivated in 

 pure water. He confirmed the occurrence of cell division in the gonidia and 

 also the formation of zoospores, these again forming new colonies of algae 

 identical in all respects with the thalline gonidia. He was able to see the 

 germinating tube from a lichen spore attach itself to a gonidium, though he 

 failed in his attempts to induce further growth. In our own country Archer 8 

 welcomed the new views on lichens, and attempted cultures but with very 

 little success. Further synthetic cultures were made by Bornet 4 , Treub 8 and 

 Borzi 6 with a series of lichen spores. They also were able to observe the 

 first stages of the thallus. Borzi observed spores of Physcia (Xanthoria) 

 parietina scattered among Protococcus cells on the branch of a tree. The 

 spores had germinated and the first branching hyphae had already begun to 

 encircle the algae. 



Additional evidence in favour of the theory of the independent origin of 

 the colourless filaments and the green cells was furnished by Stahl's 7 re- 

 search on hymenial gonidia in Endocarpon (Fig. 5). By making synthetic 



Fig. 5. Endocarpon pusillum 

 Hedw. Asci and spores, 

 with hymenial gonidia x 

 320 (after Stahl). 



Fig. 6. Endocarpon pusillum Hedw. Spore 

 germinating in contact with hymenial 

 gonidia x 320 (after Stahl). 



1 The authors quoted have been followed in their designation of the various green algae that form 

 lichen gonidia. It is however now recognized (Wille 191 3) that either Protococcus viridis Ag., Chlorella 

 or other Protococcaceae may form the universal green coating on trees, etc., and be incorporated as 

 lichen gonidia. Fleurococcus vulgaris Naeg. and Plcurococcus Naegeli Chod. are synonyms of Proto- 

 coccus viridis. In that alga there is no pyrenoid, and no zoospores are formed. 



The genus Cystococcus, according to Chodat (1913), is characterized by the presence of a pyrenoid 

 and by reproduction with zoospores and is identical with Pleurococcus vu/gartsMenegh. (non Naeg.), 

 though Wille regards Meneghini's species as of mixed content. Paulson and Hastings (1920) now 

 find that Chodat's pyrenoid is the nucleus of the cell. 



11 Woronin 1872. 3 Archer 1873, 1874, 1875. * Bornet 1873 and 1874. 



8 Treub 1873. Borzi 1875. Stahl 1877. 



