THE FORMATION OF HYPHAL FUSIONS 15 



point of fusion. The processes in the formation of a gall are 

 reminiscent of plasmogamy. Both Parasitella and Chaetocladium 

 successfully attack the homothallic species Zygorhynchus exponens. 

 Parasitella parasitises, and Chaetocladium reacts with, both (+) 

 and ( — ) mycelia of Mucor hiemalis. However, the parasitism of 

 Parasitella and Chaetocladium in respect to Absidia glauca is strictly 

 sex-limited in that Parasitella ( + ) parasitises only A. glauca ( — ) and 

 Parasitella ( — ) parasitises only A. glauca ( + ), while Chaetocla- 

 dium (+) parasitises only A. glauca ( — ). 1 In view of all these and 

 other related facts Burgeff 2 came to conclusions which may be 

 summarised as follows. The parasitism of Parasitella and Chaeto- 

 cladium probably originated by these fungi making use of their 

 sexual reactions to attack their hosts. A parasite mycelium, (-)-) 

 or ( — ), shows an affinity for both sexes of most of its heterothallic 

 hosts ; and, where this occurs, the parasite must react not to one 

 or other of the two sexual substances but to some third substance 

 excreted by both the ( + ) and the ( — ) mycelia of the hosts. Where 

 the parasitism is sex-limited, as it is in respect to the heterothallic 

 Absidiae, this third substance is not produced and the parasites 

 react to the ( + ) and ( — ) sexual substances. 



From the foregoing summary of BurgefTs investigations we see 

 that Burgeff, in attempting to throw light on the sexual and parasitic 

 fusions in the Mucorineae, has assumed as causal agents at least 

 three volatile soluble substances — a substance excreted by ( + ) 

 mycelia, a substance excreted by ( — ) mycelia, and a third substance 

 excreted by both ( -f- ) and ( — ) mycelia which is not connected with 

 sex. It is probable that, if chemical stimuli are responsible for the 

 telemorphic and zygotropic phenomena concerned with the sexual 

 and parasitic fusions of the Mucorineae, chemical stimuli are also 

 responsible for the telemorphic and zygotropic phenomena con- 

 cerned with the vegetative fusions which take place in the mycelia 

 of Ascomycetes, Basidiomycetes, and Fungi Imperfecti. Since 

 numerous hyphal fusions take place in every haploid mycelium of a 

 Higher Fungus, e.g. Coprinus lagopus, if a pair of chemical substances 



1 Chaetocladium ( — ) was not obtained and therefore no experiments could be 

 made with it. 



2 H. Burgeff, loc. cit., p. 133. 



