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Summary Symbiosis tests carried out between orchids from several geographical localities and thirty‐two Rhizoctonias isolated from orchids, non‐orchid hosts and soils of worldwide distribution, showed no evidence of any species‐to‐species relationship between orchid and fungus. Dactylorhiza purpurella established a symbiotic relationship with most fungi tested while Coeloglossum wide, Goodyera repens and Cymbidium canaliculatum, although less frequently symbiotic, established compatible infections with many isolates. Among orchids with photo‐synthetic protocorms, Epidendrum radicans was compatible with several fungi but three other species of Epidendrum together with Spathoglottis plicata and a Laeliocattleya cultivar were symbiotic only with Tulasnella calospora and certain Ceratobasidium isolates. Tulasnella calospora may be a universal orchid symbiont. Ceratobasidium cornigerum and Thanatephorus orchidicola commonly occur as root endophytes but are doubtfully symbiotic with most orchids. T. cucumeris is frequently symbiotic with Dactylorhiza purpurella but less commonly so with other orchids tested.

SUMMARYGoodyera repens Br. was either grown from seed and infected with its natural mycorrhizal endophyte, Ceratobasidium cornigerum (Bourd.) Rogers, or obtained as natural mycorrhizal plants from the field. Insoluble [^*C]carbohydrate was provided at a point source available only to the external mycorrhizal mycelium of protocorms, plantlets (up to 50 mg w wt) or plants. Protocorms and plantlets obtained ^*C via their external mycelium but plants did not, even when stressed for carbon by being kept in darkness for two weeks. There was no enhancement of growth of mycorrhizal plants when a carbon source was present in the substrate over an eight-week period. When ^^COg was fed to plant shoots to monitor movement of carbon from plant to endophyte, there was no evidence of such movement over a 7 d period.These results suggest that carbon moves only from fungus to orchid and that this movement ceases when the host reaches a certain stage of development. With respect to carbon, mature green plants of G. repens appear to be independent of their mycorrhizal association.