Kelley — 38 — Mycotrophy 



logically deducible that the only fungi available to forest trees for 

 formation of mycorrhizae would be those of the forest soil ; while 

 phycomycetes of cultivated ground are available to crop plants. Never- 

 theless, there is a physiological separation possible amongst soil fungi, 

 according to Melin (1925), whO' recognized three groups of these 

 fungi, vis., symbiophiles, saprophytes and parasites. All "mycorrhi- 

 zal fungi" are considered as symbiophiles. 



Some investigators, wishing to prove that sporophores of Russula, 

 etc., which appear on the forest floor are actually part of the mycor- 

 rhizal mycelium, have laboriously traced that mycelium from the 

 sporophore to the mycorrhiza and thereby established, so they said, 

 the identity of that particular mycorrhizal fungus. But their success 

 was denied by other investigators who asserted that attachment of a 

 sporophore to a mycorrhiza is no proof whatever that the fungus 

 concerned is mycorrhizal ; for who can say but that this sporophoric 

 fungus is not a secondary parasite? Therefore, say these later stu- 

 dents, the only thing to do is to grow the fungi in pure culture, in- 

 oculate them into sterile seedlings, and if a mycorrhiza results there 

 is positive proof of the identity of the mycorrhizal fungus. But is 

 there positive proof ? Laboratory experiments show what can happen 

 in the laboratory but not what happens in nature. A laboratory syn- 

 thesis of Boletus granulatus with pine shows by its production of a 

 mycorrhiza that this fungus is capable of such production but it does 

 not prove that mycorrhizae produced on pine in nature were pro- 

 duced by B. granulatus. They might have been produced by another 

 fungus growing on the same area. When only a single fungal species 

 has formed sporophores over the roots of pine and when that species 

 is shown by synthesis-experiment to be able to produce mycorrhizae, 

 then it can be said with justice that this species is the mycorrhizal 

 fungus in question ; but one could have come to that conclusion with- 

 out experiment. Or, to use Romell's (1939) illustration: Lacta- 

 rius delicosus has been grown on pine in the laboratory but in nature 

 it rarely if ever is found on pine. In other words, the various lines 

 of research used with reference to mycorrhizal fungi all help to iden- 

 tify the fungi ; but the question of identity is after all not of major 

 importance. 



In earlier days of mycorrhizal research, it was thought that my- 

 corrhizae were produced on trees by basidiomycetes and that herbs 

 in general lack mycorrhizae ; but with greater development of micro- 

 scope and technique it is known that all major fungal groups furnish 

 mycorrhizal fungi. We shall consider them in the usual systematic 

 order. 



