542 BOTANICAL GAZETTE [December 



algae to the lichens. Artari grew Cystococcus humicola, obtained from the 

 thalli of Xanthoria parietina and Gasparrinia murorum, in pure cultures on 

 complicated media containing mineral salts or organic compounds, or in some 

 instances both the salts and the organic matter. He found that the algae grew 

 luxuriantly on the media containing organic matter and were dark green in color 

 when grown in absolute darkness or in light with C0 2 excluded. On media 

 containing mineral salts but no organic matter, the algae grew under similar 

 conditions, but not so well. These results square beautifully with those of 

 Tobler, the one worker giving special attention to the lichen, and the other 

 to the alga which lives in the thallus of the same lichen. The two researches 

 show conclusively that the alga which lives in the thallus of Xanthoria parietina 

 can obtain its organic matter as well as the mineral salts from the lichen, pro- 

 vided the latter grows on some other substratum than rocks that contain no 

 organic matter. 



Beyerinck, Bouilhac, Etard, Klebs, and others have obtained results 

 with algae somewhat similar to those of Treboux and Artari, two of the 

 authors working on a blue-green alga (Nostoc). Trentepohlia umbrina grows 

 in the bark of trees, sometimes with lichen thalli and sometimes alone. In 

 fact this alga bores into the bark of trees or effects an entrance through minute 

 openings in the periderm, probably either because abundant light is not favor- 

 able to its best development, or because closer relationship with the organic 

 matter of the periderm is more essential to its development than bright light. 

 This alga would be an especially favorable object for such investigations as 

 those noted above. 



Tobler also started cultures of Xanthoria on gelatin and transferred them 

 to a liquid medium which contained none of the carbon compounds needed by 

 the alga except what was in the air, and the lichen grew somewhat. The alga 

 grew in the same liquid medium and was of normal appearance. In trans- 

 ferring the lichen, particles of the gelatin were unavoidably carried over. After 

 these were probably consumed by the lichen, the alga was introduced into the 

 culture with the lichen and grew well, but was colorless. This, he thinks, 

 indicates that the lichen had assimilated the acid which the alga needed as a 

 source of carbon, probably oxalic acid. He says that gelatin is not a source 

 of carbon for the alga, so that his conclusions would not be invalidated even 

 if the lichen had not extracted all of the gelatin from the medium before the 

 alga was introduced. 



The lichen hyphae soon entwine some of the algal cells in the culture, and 

 thus the parasitic, or at least a symbiotic, relationship is established. Tobler 

 believes that the lichen obtains carbon from the alga, while the latter replaces 

 it by extracting carbon from the oxalic acid contained in the tissues of the 

 lichen. This means a mutual exchange of food materials between the lichen 

 and the alga, at least as a probability. His investigation seems to indicate 

 that the alga growing in Xanthoria parietina and those found in several other 

 lichens very probably obtain their carbon from the lichen, the latter in turn 



