K.— BOTANY. 197 



may be difficult to form a conception of a race with four sexes, but a race 

 requiring four or more food substances in preparation for the fruiting 

 period is a matter of common experience. Let the four characters, known 

 as A, a, B and b, which these fungi have been shown to inherit on mendelian 

 lines, represent each the capacity of rapidly extracting from the substratum 

 some essential food, and let every spore contain, as it is known to do, 

 either A or a and either B or b ; then the requisite food supply is assured 

 only when AB and ab, or Ab and aB have pooled resources. In other 

 words, for an AB strain the limiting factors are the scarcity of a and b, 

 while the development of an ab strain is restricted by poverty in respect 

 of A and B. If different sporophores develop a different arrangement of 

 limiting factors, the otherwise astonishing fact that mycelia from all the 

 spores of one heterothallic sporophore may be fertile with those from 

 all the spores of another is readily understood. 



The higher Basidiomycetes are wholly lacking in sexual organs, and 

 it is impossible to judge whether the heterothallic condition arose, as in 

 Ascomycetes, while these were still extant. A further study of the 

 heterothallic rusts may throw light on this interesting question. 



In Ascomycetes account has to be taken of so many peculiar features 

 that one hesitates to suggest any correlation between them. To those 

 who accept the observation of Harper^^ and his many successors that a 

 nuclear fusion in the oogonium is followed by a fusion in the ascus, the 

 simultaneous occurrence of heterothallism and sexuality is at least 

 suggestive. But in the present state of our knowledge it is no more, 

 and the suggestion may lead to another of the blank walls with which the 

 study of fungi is beset. Particularly to be desired is the full investigation 

 of a heterothallic form in which the entrance of male nuclei into the 

 oogonium still occurs. Pyronema confluens and Pyronema domesticum 

 are uncompromisingly homothallic, male and female organs and normal 

 fruits being found in single spore culture. There is some hope of Ascobolus 

 magnificus or Ascobolus carbonarius. 



Since heterothallism occurs in all the main groups of Basidiomycetes 

 and Ascomycetes, it may be inferred that its origin is remote, and the 

 question arises whether the phenomenon, as elucidated in these fungi, 

 bears any relationship to the heterothallism of the Mucorales. In Mucor 

 and its allies the branches of (+) and (— ) mycelia grow towards one 

 another and become attached, the procedure up to this stage being very 

 similar to that in Humana and, I should judge, in the Hymenomycetes 

 also. The result of contact, however, is the development of sexual organs 

 at the point of union. This differs from Hicmaria, in which only female 

 organs are formed, and those on a neighbouring branch ; and from the 

 Hymenomycetes, in which sexual organs are not produced. Moreover, 

 in the mucors, open communication does not occur between (+) and (— ) 

 strains till their gametangia are mature, and, if the distinction between 

 them be nutritive, the nutritional deficiencies of each mycel'um must at 

 first be supplied by diffusion. It is true that the archicarps of Humaria 

 develop up to a point without mycelial fusion, but in Mucor the presence 

 of two mycelia is necessary before gametangia appear. As an argument 

 for the sexual nature of the (+) and (— ) strains in the Mucorales, Satina 

 and Blakeslee" have lately shown that, with the KMnO., and Manilov 



