40 CONSTITUENTS OF THE LICHEN THALLUS 



very slow and very feeble until he added to the culture-medium a solution 

 of malt-extract which contains peptones and sugar. Very soon he obtained 

 an active development of the gonidia, and they multiplied rapidly by 

 division' as in the lichen thallus. This proved to him conclusively the great 

 advantage to the algae of an abundant supply of nitrogen. 



Artari in his work has demonstrated that there are two different physio- 

 logical races of green algae: (i) those that absorb peptones — which he 

 designates peptone-algae — and (2) those that do not so absorb peptones. 

 He tested the cells of Cystococcus humicola taken from the thallus of Physcia 

 parietina, and found that they belonged to the peptone group and were 

 therefore dependent on a sufficiency of nitrogenous material to attain their 

 normal vigorous growth. It was also discovered by Artari that the one 

 race can be made by cultivation to pass over to the other: that ordinary 

 algae can be educated to live on peptones, and peptone-algae to do without. 



We learn further from Beyerinck's researches that Ascomycetes, the 

 group of fungi from which the hyphae of most lichens are derived, are 

 what he terms ammonia-sugar fungi; that is to say, the hyphae can 

 abstract nitrogen from ammonia salts and, with the addition of sugar, can 

 form peptones. The lichen peptone-algae are thus evidently, by their 

 contact with such fungi, in a favourable position for securing the nitro- 

 genous food supply most suited to their requirements. In their deep-seated 

 layers, they are to a large extent deprived of light, but it has been proved 

 by Artari^ in a series of culture experiments extending over a long period, 

 that the gonidia of Xanthoria parietina remain green in the dark under 

 very varied conditions of nutriment, though the colour is distinctly fainter. 



Recently Treboux' has revised the work done by Artari and Beyerinck 

 in reference to Cystococcus humicola. He denies that two physiological races 

 are represented in this alga, the lichen gonidia, in regard to the nitrogen 

 that they absorb, behaving exactly as do the free-living forms of the species. 

 He finds that the gonidium is not a peptone-carbohydrate organism in the 

 sense that it requires nitrogen in the form of peptones, inorganic ammonia 

 salts being a more acceptable food supply. Treboux concludes that his 

 results fayour the view that the gonidia are in an unfavourable situation for 

 receiving the kind of nitrogenous compound most advantageous to them, 

 that they are therefore in a sense "victims" of parasitism, though he 

 qualifies the condition as being a lichen-parasitism or helotism. This view 

 does not accord with Chodat's* results: in his cultures of gonidia he 

 observed that with glycocoll or peptone, which are nearly equivalent, they 

 developed four times better than with potassium nitrate as their nitrogenous 

 food, and he concluded that they assimilated nitrogen better from bodies 

 allied to peptides. 



1 See p. 56. 2 Artari 1902. 3 Treboux 1912. * Chodat 1913. 



