188 SECTIONAL ADDRESSES. 



been found which will produce sex intergrades^^ or transform ( — ) into (+) 

 or (+) into (— ) mycelia. 



It is, I think, to be regretted that the term heterothallic has recently 

 been used to indicate the condition of dioecism in the gametophyte. The 

 old sex terms are adequate for this purpose and there is a need, which 

 heterothallism admirably fulfils, for a term appropriate to a thallus having 

 two or more physiologically distinct but morphologically similar strains, 

 whether the difference between them be sexual or no. I propose to employ 

 the word in that sense this morning. 



It is a curious point in the Mucorales that, while heterogametangia 

 are common, these are never found in correlation with the heterothallic 

 condition. In heterothallic forms the two gametangia of a pair may 

 differ in size, but both large and small gametangia are borne on the same 

 mycelium. There is, perhaps, a faint suggestion here that some factor 

 other than sex may be at work in determining the heterothallic condition. 



Basidiomycetes. 



When we turn to the higher fungi, which are characterised by the 

 possession of a septate mycelium, we find one of their most striking 

 vegetative characters to be a tendency to fusion between the hyphse. A 

 branch will grow out, wander a little way, turn and fuse with the parent 

 filament again ; it will e^'en do this two or three times at short intervals. 

 A hypha will undergo dichotomy, and a cross connection will unite the 

 diverging branches. Stranger still, germ tubes from several distinct 

 conidia will flow into one another, and the composite mycelium thus 

 produced will continue its ordinary development. Presumably the 

 stimulus responsible for such unions is nutritive, but there is at present no 

 evidence that they are conditioned by general starvation. Certainly they 

 complicate the question of what may be regarded as an individual in the 

 fungi, since, where mycelial fusions have taken place, nuclei from several 

 sources may be intermingled in the same cell. In the smuts, mycelia from 

 three or four species have even been described^* as involved in the same 

 group of fusions, and in Ascomycetes two species may apparently take 

 part^^. 



It is perhaps unfortunate that the suggestion that some of these 

 mycelial fusions are sexual was first made in the Hymenomycetes, where 

 sexual organs, the ordinary criteria of sex, are wholly lacking. Kniep^® 

 from 1915 onwards, and Bensaude^' independently in 1918, described the 

 union of two mycelia as a necessary preliminary to the formation of the 

 sporophore in certain species, and characterised such a mycelial fusion as 

 a sexual act. 



On the germination of the basidiospore in, for example, Coprinus 

 fimetarius, a multinucleate filament is put out and grows for a time, dividing 

 into uninucleate cells. This is the primary mycelium. All primary 

 mycelia are similar in appearance, but they are of two kinds, which, here 

 also, are distinguished as (+ ) and ( — ). Should a (+ ) and a ( — ) mycelium 

 meet, fusions occur, with the formation of secondary mycelium on which 

 sporophores may develop, and which is characterised by the presence of 

 clamp-connections. In Covrinus fimetarius oidia are liberated from the 



