42 CONSTITUENTS OF THE LICHEN THALLUS 



the green cells; it is also partly contributed by the fungus as a product of 

 its metabolism. A proof of this is afforded by Dufrenoy 1 : he found a 

 Parmelia growing closely round pine needles and even sending suckers into 

 the stomata. He covered the lichen with a black cloth and after seven weeks 

 found that the gonidia had remained very green. That growth had not 

 been checked was evidenced by an unusual development of soredia and 

 of spermogonia. Dufrenoy describes the condition as a parasitism of the 

 algae on the fungus which in turn was drawing nourishment from the 

 pine needles. 



Artari 2 has proved that lichen gonidia can obtain carbohydrates from 

 the substratum as well as by photosynthesis. He cultivated the gonidia of 

 Xanthoria parietina and Placodium murorum on media which contained 

 organic substances as well as mineral salts, while depriving them of atmo- 

 spheric carbon-dioxide and in some cases of light also. The gonidia not 

 only grew well but, even in the dark, they remained normally green, a 

 phenomenon coinciding with Etard and Bouilhac's 3 experience in growing 

 Nostoc in the dark: with suitable culture media the alga retained its colour. 

 Nostoc also grows in the dark in the rhizome of Gunnera. Radais' 4 experi- 

 ments with Chlorella vulgaris confirmed these results. On certain organic 

 media growth and cell-division were as rapid in the dark as in the light, 

 and chlorophyll was formed. The colour was at first yellowish and the full 

 green arrived slowly, especially on sugar media, but in ten days it was 

 uniform and normal. 



When making further experiments with the alga, Stichococcus bacillaris, 

 Artari 8 found that it also grew well on an organic medium and that grape 

 sugar was the most valuable carbonaceous food supply. Chodat 6 also found 

 that sugar or glucose was a desirable ingredient of culture media. 



Treboux 7 , in his work on organic acids, has also proved by experimental 

 cultures with a large series of algae, including the gonidia of Peltigera, that 

 these green plants in the absence of light and in pure cultures would grow 

 and form carbohydrates if the culture medium contained a small percentage 

 of organic acids. The acids he employed were combined with potassium 

 and were thus rendered neutral or slightly alkaline; acetate of potash 

 proved to be the most advantageous compound of any that was tested. 

 Amino-acids and ammonia salts were added to provide the necessary 

 nitrogen. Oxalic acid and other organic acids of varying composition are 

 peculiarly abundant in lichen tissues and may be a source of carbon supply. 

 Marshall Ward 8 has found calcium carbonate crystals in the lower air- 

 containing tissues of Strigula complanata. 



Treboux finally concluded from his researches that just as. fungi can 



1 Dufrenoy 1918. 2 Artari 1899. 8 Etard and Bouilhac 1898. 4 Radais 1900. 



s Artari 1901. 8 Chodat 1913. 7 Treboux 1905. 8 Marshall Ward 1884. 



