Kelley — 140 — Mycotrophy 



Freisleben's statements about Vaccinium have already been noted : 

 he showed that in this genus there is no general shoot and seed in- 

 fection, and accordingly no cyclic symbiosis can exist. In V. macro- 

 carpon, according to Bain (1937), "Systemic infection of the type 

 attributed tO' Phoma radicis by some investigators could not be found 

 either in seedlings or in prepared slides from field grown material. 

 The hypothesis of systemic infection by mycorrhizal fungi and its 

 obligate relationship to root formation in the heath family was 

 examined critically and it was shown that the hypothesis fails to con- 

 form to observed facts in some important respects, for example, in 

 100,000 cultures of cranberry fruits made by the U.S.D.A. in the 

 last 30 years not even a single culture developed Phoma radicis." 



The casual nature of the mycorrhizal symbiosis is made evident 

 once again by these studies. Apparently there is no obligate symbiosis 

 in ericads in the sense that the ericad could not live without the fungus, 

 neither that the two are habitually associated ; but yet the fact remains 

 that in the ericads' usual habitat, conditions exist which favour mycor- 

 rhizal association with an endophyte, and such exists. Perhaps in 

 unusually favourable habitats there is a general systemic infection. 



The Orchids: — Next may be considered the orchids, in reference 

 to which there has been much argument as to their possible obligate 

 mycotrophy. As to the possibility of its occurrence in roots and 

 rhizomes of orchids, a denial was made a century ago by Reissek 

 (1847) : "The regularity and Constance with which fungal formation 

 occurs in orchids must be considered a characteristic and vital pheno- 

 menon, but just as phanerogams can be propagated without seed, so 

 can orchids be produced without root-fungi." 



In the orchid Gastrodia, Kusano (1911) concluded that the plant 

 is dependent on its fungal endophyte because only in association did 

 the orchid thrive and bloom well. But Curtis (1937) denies that 

 there are any specific endophytes with orchids. He said : "There is 

 an apparent correlation between ecological habitat and fungus type, 

 rather than between orchid species and fungus." Bog orchids all had 

 the same fungus indiscriminately but some orchids had dilTerent 

 fungi dependent upon whether they grew in bog or prairie. Several 

 strains of RJiisoctonia were isolated from the same orchid. Hence 

 Curtis concluded that there is no specific mycorrhizal fungus, but 

 any species found in the habitat may form mycorrhizae. This is the 

 same principle of fortuitous mycorrhizal formation that seems gener- 

 ally to be true. 



While speaking of orchids as a unit, it is evident that, like other 

 sorts of biological phenomena, they consist of various discrete entities. 



