BASIDIOMYCETES 419 



Between two haploid, dynamically different, primary mycelia, there 

 occurs pseudogamous plasmogamy either between two hyphae, or 

 between a hypha and a germinating secondary spore, or between two sprout 

 cells, or between two germinating secondary spores, or it may be shifted 

 forward into the basidiospores (as in the Ustilaginales, particularly in 

 Tilletia Tritici), and take place between two basidiospores. Their prod- 

 ucts are chiefly binucleate cells which develop into neutral, diploid, 

 secondary mycelia which are not differentiated from the haploid mycelia 

 in their fundamental relationships, and which in the Tremellales may 

 proceed to the formation of secondary imperfect forms. Under definite 

 external conditions, these secondary mycelia (in the higher forms) pro- 

 ceed to the formation of fructifications upon which the basidia arise in 

 definite layers; in these caryogamy occurs, directly followed by meiosis 

 and segregation of sex. The tetracytes (basidiospores) germinate again 

 to primary haploid, dynamically distinct mycelia. 



The second, homothallic scheme of development takes a somewhat 

 simpler form, as in meiosis, the segregation of sex is omitted and the 

 haploid mycelia are neutral and equivalent to each other. Although as 

 yet no forms of this type have been investigated cytologically, there can 

 be no doubt that in the ideal case, their development will proceed accord- 

 ing to the following scheme: 



± Basidiospores 



P 



-Uninucleate ± imperfect former Binucleate + imperfect form; 



I — Uninucleate ±impertect torm^ 

 "Uninucleate -fmvceliun r > 



R 



Uninucleate ± myceliunr^— — ^Binucleate ± mycelium ^— ^Fructification -+Basidia 



Diagram XXVI. 



Mutatis mutandis, what has been said for the first, holds for this scheme, 

 only the binucleate hyphae do not result from a pseudogamous plasmo- 

 gamy between two vegetative hyphae but apparently from the resorption 

 of a septum or from a nuclear division which is not followed by septal 

 formation in a single hypha. 



Just as in the Coprinus fimetarius type, plasmogamy is shifted forward 

 under certain conditions into the basidiospores (in Tilletia Tritici, Fig. 

 399) so this critical nuclear division, which is not followed by septal for- 

 mation, may take place in the basidiospores. Therefrom results the 

 following scheme which will be indicated as the third, or Gasteromy- 

 cetous type: 



i — Binucleate imperfect form C R 



/ 



^ Binucleate! myceliurii->Fructification->Basidia-»uninucleate then-»binucleate basidiospores 



Diagram XXVII. 



