LICHEN GONIDIA 45 



used up by the fungus and any dead gonidia are likewise utilized for food 

 supply. It is also taken for granted that the fungus takes advantage of the 

 presence of humus whether in the substratum or in aerial dust. In such 

 slow growing organisms, there is not any large demand for nourishment on 

 the part of the hyphae: for many lichens it seems to be mere subsistence 

 with a minimum of growth from year to year. 



C. Symbiosis of other Plants 



The conception of an advantageous symbiosis of fungi with other plants 

 has become familiar to lis in Orchids and in the mycorhizal formation on 

 the roots of trees, shrubs, etc. Fungal hyphae are also frequent inhabitants 

 of the rhizoids of hepatics though, according to Gargeaune^ the benefit to 

 the hepatic host-plant is doubtful. 



An association of fungus and green plant of great interest and bearing 

 directly on the question of mutual advantage has been described by 

 Servettazl In his study of mosses, he was able to confirm Bonnier's^ 

 account of lichen hyphae growing over such plants as Vaucheria and 

 the protonema of mosses, which is undoubtedly hurtful; but he also found 

 an association of a moss with one of the lower fungi, Streptothrix or 

 Oospora, which was distinctly advantageous. In separate cultivation the 

 fungus developed compact masses and grew well in peptone agar broth. 



Cultures of the moss, Phascuni cuspidatiim, were also made from the 

 spores on a glucose medium. The specimens in association with the fungus 

 were fully grown in two months, while the control cultures, without any 

 admixture of the fungus, had not developed beyond the protonema stage. 

 Servettaz draws attention to the proved fact that, in certain instances, 

 plants benefit when provided with substances similar to their own decay 

 products, and he considers that the fungus, in addition to its normal gaseous 

 products, has elaborated such substances, as acid products, from the glucose 

 medium to the great advantage of the moss plant. 



A symbiotic association of Nostoc with another alga, described by 

 Wettstein", is also of interest. The blue-green cells were lodged in the 

 pyriform outgrowths of the siphoneous alga, Botrydiuvi pyriforme Kiitz., 

 which the author of the paper places in a new genus, Geosiphon. The 

 sheltering Nostoc symbioticum fills all of the host left vacant by the plasma, 

 and when the season of decay sets in, it forms resting spores which migrate 

 into the rhizoids of the host, so that both plants regenerate together. 



Wettstein has compared this symbiotic association with that of lichens, 

 and finds the analogy all the more striking in that the membrane of his new 

 alga had become chitinous, which he thinks may be due to organic nutrition. 



' Gargeaune 191 1. * Servettaz 1913. * See p. 65. * Wettstein 1915. 



