194 SECTIONAL ADDRESSES. 



fertile perithecia have been reported along the line of junction of two 

 strains, but full details are not available. 



In 1926 Shear and Dodge** described a new genus, Neurospora, the 

 red bread mould, which they classified among the Hypocreales in the 

 neighbourhood of Melanospora. N. tetrasperma has four binucleate spores 

 in the ascus ; in N. sitophila each of the eight ascospores contains one 

 nucleus. Grown in culture N. tetrasperma readily produced ascocarps, 

 while, in N. sitophila, perithecia appeared only at the junction of (+) and 

 ( — ) mycelia. Further, mycelia from the occasional uninucleate spores of 

 Neurospora tetrasperma were heterothallic like those from the spores of 

 the eight-spored species. Dodge*^ and his colleague Wilcox,*® who 

 studied N. sitophila, concluded that the character distinguishing the 

 (+) and (— ) strains was carried by the nuclei and found evidence that 

 its distribution took place in the second division in the ascus. Dodge*'' 

 succeeded in intermingling the mycelium of N. sitophila with that of the 

 heterothallic form of N. tetrasperma, and in obtaining material with some 

 of the characters of each. Unfortunately no information is available as 

 to the sexual apparatus of these fungi, or of the part it plays, if any, in the 

 relation of (+) and (— ) strains. 



This relation is described by Dodge, and by most other workers on 

 heterothallism in the Ascomycetes, as in the Basidiomycetes, in terms of 

 sexual difference. I have tried to state their facts without theoretical 

 implication. 



Lately some work has been in progress in my laboratory at Birkbeck 

 College on the coprophilous species, Humaria granulata, in which Prof. 

 Blackman and I,** some twenty-two years ago, described the archicarp, 

 terminating in a globose oogonium, and giving rise to ascogenous hyphse 

 without the intervention of an antheridium. I am not sure, in adducing 

 the case of Humaria, whether I am bringing forward that additional 

 term which sometimes solves an equation, or only making an insoluble 

 equation more complex. 



We found that the mycelia of Humaria, in single spore culture, were of 

 two kinds, and that ascocarps developed only along the line of junction of 

 (+) and ( — ) infections. So far the case was an ordinary one of hetero- 

 thallism, but microscopic examination showed that both (+) and ( — ) 

 mycelia bear well-grown female organs, though these produce ascogenous 

 hyphse only where (+) and ( — ) strains have met. The contact of the 

 mycelia is followed by fusions between their branches, and it is in the 

 neighbourhood of such points of union that successful archicarps are 

 found. Transverse walls do not at first appear in the archicarp, so that 

 little difficulty is presented to the passage of nuclei from both mycelia to 

 the oogonium. 



It is impossible to regard as differing in sex these two mycelia 

 which both bear normal, though apogamous, female organs ; and it is 

 therefore inevitable, in Humaria at any rate, to seek some explanation 

 of heterothallism which does not invoke sexual difference. The most 

 promising alternative appears to be a difference in nutrition. If we can 

 induce Humaria to fruit on synthetic agars, we hope to make a direct 

 test of this hypothesis. Meantime there is other work from which indirect 

 information can be obtained. 



i 



