190 SECTIONAL ADDRESSES. 



appears to depend in these cases on two sets of allelomorphic factors, 

 A, a and B, b. Each spore, and hence each primary mycelium, carries 

 a member of either pair, so that they may be AB, Ab, aB or ab. Secondary 

 mycelium develops only when the combination AaBb has been achieved. 

 In Coprinus lagopus^^ half the basidia carry the four spores AB, Ab, aB and 

 ab, while twenty-five per cent, develop two AB and two ab spores, and the 

 remainder two Ab and two aB. Tins is what might be expected if the 

 characters A, a, B and b are transmitted on mendelian lines, and if the 

 allelomorphs A, a and B, b are independently inherited. One cannot but 

 admire the delicate and persevering work involved in separately collecting 

 and germinating the spores from so minute an object as a basidium, thanks 

 to which the mode of transmission of these characters seems to have been 

 fully established. 



Their significance, however, is by no means so clear. It is customary 

 among the workers in this field, following the analogy of the Mucorales, 

 to refer to the distinction between (+) and ( — ), or between AB, Ab, aB 

 and ab strains as a sexual difference ; but, if we accept this point of view, 

 we must greatly extend our notions of sex. Not only must we accept the 

 occurrence of four sexes, but we must assume that sex is variable, male 

 or female strains spontaneously becoming hermaphrodite. And even that 

 is not sufficient. In a number of species mycelia from all the spores of 

 distinct ' sexes ' on one sporophore may be perfectly fertile with those 

 from all the spores on another sporophore^®. In other words, mycelia of 

 one sex achieve fertile unions not only with mycelia of the opposite sex, 

 but with mycelia of the same sex, provided that these are derived from a 

 different source. And yet ' sex ' in these fungi is only recognisable as a 

 capacity for selective fusion. In plants possessing recognisable sexual 

 organs, it might be possible to unravel such a tangle, and we may turn, 

 therefore, with special interest, to groups less remote than the Hymeno- 

 mycetes from normal sexuality. 



In the smuts it has long been known that the basidiospores or their 

 products fuse readily in pairs ; Dangeard,'^' in 1894, first described the 

 union of two nuclei in the young brand spore ; later it was realised that 

 nuclei first became associated in the paired basidiospores and that the 

 intervening mycelium consisted of binucleate cells. In Ustilago anther- 

 arum and other species,'^" as in the Hymenomycetes, two or more strains 

 may exist, and fusions are not indiscriminate but between cells of opposite 

 strain. The formation of strains between which fusion does not occur may 

 be induced by cultivation on media rich in albuminous compounds, and, 

 conversely, the tendency to fuse may be enhanced by an ample supply of 

 oxygen or by scarcity of food. 



In the rusts, thanks to the work of Blackman^' in 1904, Christman^" 

 in 1905 and subsequent investigators, we have a pretty full knowledge of 

 the morphology of the reproductive apparatus. In the eu forms, which 

 possess a complete life cycle, both uredospores — the accessory spores of 

 the sporophyte— and teleutospores are produced on a mycelium of 

 binucleate cells. Nuclear fusion takes place in the teleutospore cell, which 

 is the young basidium, meiosis follows, and four uninucleate basidio- 

 spores are shed. The basidiospore, on germination, gives rise to a mycelium, 

 the cells of which contain each a single nucleus, and some of them, forming 



