GENERAL SUMMARY 301 



while the other ninety-nine remain sterile and exhaust themselves in 

 supplying the more fortunate mycelium with the necessary food-stuflfs. 



The biological advantage to a hymenomycetous species in forming 

 compound mycelia which act as units in the production of fruit-bodies 

 lies in these considerations : (1) that every mycelium, before it can 

 produce a fruit-body, must attain a certain mass ; (2) that, in a given 

 substratum, it must often happen that many mycelia are present at 

 one and the same time and that each of them by itself never attains a 

 sufficiently large mass to produce a fruit-body ; and (3) that, by means 

 of hyphal fusions and a union of the mycelia, competition which would 

 result in general sterility is replaced by co-operation which results in a 

 fertility that is highly advantageous for the perpetuation of the species. 

 The components of a compound mycelium which genetically remain 

 sterile may be compared to the neuter bees in a hive which never repro- 

 duce themselves but are a prime factor in securing the persistence of the 

 hive-bee species, Apis mellifica. 



In the Pyrenomycetes, e.g. Nectria Solani, Trichoderma lignorum 

 (a stage of Hypocrea rufa), and Pleurage anserina ; in the Discomycetes, 

 e.g. Ascobolus magnificus ; and in the Fungi Imperfecti, e.g. Colletotrichum 

 trichellum : hyphal fusions take place between adjacent mycelia derived 

 from different spores, with the result that compound mycelia are formed. 

 Doubtless such compound mycelia act as social units in the production 

 of fruit-bodies, thus behaving in a manner comparable with that already 

 described for the Hymenomycetes. 



Hyphal fusions in the Hymenomycetes are of importance in the five 

 following ways : (1) they convert every young mycelium into a network 

 of such a kind that food materials can be conducted through it to fruit- 

 bodies, sclerotia, etc., in diverse directions ; (2) in heterothallic species, 

 they place two mycelia of opposite sex in continuity, thus making 

 possible the association of their nuclei in conjugate pairs ; (3) in hetero- 

 thallic species, they convert the haploid mycelia into a network of such 

 a kind that, when a haploid mycelium is being diploidised by nuclei 

 derived from another haploid mycelium, the nuclei can travel through 

 the mycelium which is being diploidised by numerous and varied routes ; 

 (4) they convert all mycelia, whether haploid or diploid, into a network 

 of such a kind that a mycelium, when injured by the breaking of some of 

 its hyphae, still remains a unit and can act as such in the production of 

 a fruit-body ; and, finally, (5) in any single species, whether honio- 

 thallic or heterothalHc, they permit of any number of adjacent mycelia, 

 whatever may be their sexual state, uniting to form a compound 

 mycelium and thus acting as a social unit in the production of fruit- 

 bodies and spores. 



With a view to throwing further light on the relation between hyphal 

 fusions and sexual phenomena in the Hymenomycetes, some additional 

 remarks have been made. 



