Lecture X — 137 — Obligate Symbiosis 



by many species of Hygrophorns and Agaricaceae. In all cases the 

 variations in the florula seem correlated with the sort of humus 

 present. 



Special Gases: — Such examples could be multiplied but the 

 objection is raised that finding of sporophores under certain trees 

 is not a priori proof that these fungi are mycorrhizal symbionts of 

 the trees ; neither does tracing of mycelial connection between the two 

 constitute proof. But Romell (1930) offers the interesting observa- 

 tion that Lactarius delicosus not only constantly occurs with Picea in 

 the region of Stockholm, but that this species disappeared upon re- 

 moval of the few spruce trees that stood in a mixed pine stand. On 

 the other hand, Dittrich (1923) said that L. delicosus is found in 

 great quantities under thick-set spruce trees when the trees are young, 

 but disappears completely after the trees have reached a certain age. 

 An interesting case is cited by Mattirolo (1934&) in that Populus 

 canadensis was introduced into Italy from America about 100 years 

 ago : Tuber Borchii is found on this poplar and, as the fungus is 

 recorded also from California, may have been introduced into Italy 

 on introduced Amentaceae. Further, Melin (1922) said that Boletus 

 elegans appeared in Sweden only after introduction of larch. 



Fungi and Herbs: — Associations of fungi with herbs are like- 

 wise noted. Thus Colla (1931) found two species of fungi associated 

 with Dryas ocfopetala. Fraser (1931) made "the first report of an 

 obligate association of an annual herb with a mycorrhizal fungus" 

 in two species of Lobelia. In a study of German species of Polygala, 

 Heinricher (1900) concluded that some species at least of this 

 genus can scarcely be considered as obligate mycorrhizal plants, for 

 he was able to grow the plants in culture without mycorrhizae. For 

 the orchids, Kusano (1911) thought that Gasfrodia was unable to 

 flower without infection as indicated by pot experiments. Obligate 

 symbiosis is indicated by Porter (1942), who reports that Rhisoc- 

 tonia mucoroides is probably the specific endophyte of Zeuxinc 

 stratcumatica since this fungus is found both in Florida and in Java. 



Obligatism and Nutrition: — Without multiplying examples, the 

 argument is the same in all cases : Certain fungal fruiting bodies are 

 found habitually in the neighbourhood of certain higher symbionts, 

 and by careful examination a mycelial connection can be traced be- 

 tween the two. The objection is raised that here there is no necessary 

 obligate relation since it is possible, perhaps, to grow the two apart 

 in sterile culture. The fact remains, however, that the two symbionts 



