The existence of oidia has long been known and their structure and development have been described using the light microscope (Brodie, 1936). They are formed asexually, are uninucleate and usually borne in clusters on the monokaryotic mycelia of many basidiomycetes. Oidia of different species may have different functions. In some species they readily germinate to form monokaryotic mycelia. In many coprophilous species they do not germinate (Falck, 1902), and they probably act as fertilizing agents or spermatia. The oidia of Psathyrella coprophila Watling (Watling & Jurand, 1971)~ which were used in this investigation, do not germinate. They are borne on oidiophores and cohere in a droplet of mucilage.The surface structure of oidia is of interest in studies of the relationship between the hyphae and the oidia. Hyphae of many species are known to react to an oidial stimulus by growing towards and fusing with oidia (Bistis, 1970; Kemp, 1970). The directed growth and fusion between hyphae and oidia is either followed by dikaryotization or by vacuolation and death. Dikaryotization indicates that two isolates belong to the same species. Homing followed by a lethal reaction indicates that two isolates are closely related. The physiological basis for the recognition of oidia by hyphae has not yet been resolved and a study of the oidial surface might be relevant to this problem.From this preliminary study it is suggested that the surface of the oidia of PsathyreZZa coprophila has numerous filamentous appendages which may be embedded in a layer of mucilage. A similar surface structure has been described in certain bacteria. In some respects the filamentous appendages appear to resemble bacterial fimbriae (pili) (Duguid, Smith, Dempster & Edmunds, 1955; Brinton, 1969, and the mucilaginous coat in which they are embedded may be similar to bacterial capsules (Duguid, 1951). METHODSPsathyrella coprophila was chosen because it produces abundant oidia in culture. Monokaryotic mycelia, from basidiospores of the type specimen which is deposited in the herbarium of the Royal Botanic Garden, Edinburgh, were grown on agar plates at 20 "C on horse-dung extract agar (Lange, 1952). The oidia were harvested when the mycelium almost covered the agar surface.For electron-microscope preparations the oidia were harvested with 0.8 I yo (w/v) NaCl.About 4 ml of the saline solution was poured into each plate, the oidia suspended by gentle rocking and the suspension centrifuged at 3000 rev./min for 5 min to concentrate the oidia into a pellet. Three fixation methods were used:(i) Fixation with OsO, (I %, w/v) according to Palade (I952), but with addition of 0.045 mg sucrose/ml of fixative, for I h at room temperature;

SUMMARYEvidence is given for a trifactorial system of incompatibility in an isolate identified as Psathyrella coprobia. The three factors are designated A, B and C. They are thought to be inherited independently. The function of factor A is most probably the same as that of factor A of bifactorial species. Factor B is concerned with the initiation of fruit body primordia, while all three factors must be heterozygous for the occurrence of nucleár migration and the formation of mature fruit bodies.