82 TEXT-BOOK OF FUNGI 



(6) In heterothallic species, strains have been found 

 which, from their failure to react with -+- and strains 

 of the same form, have been called ' neutral,' and a similar 

 neutrality may be induced by cultivation under adverse 

 conditions. 



(7) In all species of both groups in which the process 

 of conjugation has been carefully followed, the swollen 

 portions (progametes) from which the gametes are cut off 

 do not grow towards each other, as currently believed, but 

 arise from the stimulus of contact between more or less 

 differentiated hyphae (zygophores), and are from the outset 

 always normally adherent. 



(8) In some species the zygophores have been demon- 

 strated to be mutually attractive (zygotactic). 



(9) In the heterogamic subdivision of the homothallic 

 group, a distinct and constant differentiation exists 

 between the zygophoric hyphae and the gametes derived 

 from them, but in the remaining homothallic forms and in 

 all heterothallic forms no such differentiation is apparent. 



(10) A process of imperfect hybridisation will occur 

 between unlike strains of different heterothallic species in 

 the same or even in different genera, or between a homo- 

 thallic form and both strains of a heterothallic species. 



(n) By taking advantage of this character it has been 

 possible to group together in two opposite series the strains 

 of all the heterothallic forms under consideration. 



(12) When thus grouped the or less luxuriant will 

 be in one series, while the + or more luxuriant will be in 

 the other. 



(13) From the foregoing observations it may be con- 

 cluded : 



.(a) That the formation of zygospores is a sexual process; 



