SIGNIFICANCE OF CONJUGATE NUCLEI 285 



the mixed arrangement of uninucleate {A) and (a) cells in the 

 young aecidium at the moment when the (.4) and (a) cells there 

 present co-operate in pairs to form basal cells containing conjugate 

 nuclei. 



Of the two hypotheses suggested above I am inclined to fav^our 

 the second because, in comparison with the first, it seems simpler 

 and more in accordance with the process of diploidisation as known 

 in the Hymenomycetes. 



The most important part of Hypothesis No. 2 seems to me to 

 be the assumption that, in the Uredineae, one or a few nuclei 

 derived from a haploid mycelium of one sex can move into and 

 through a haploid mycelium of opposite sex, multiply in number 

 there, and so provide one member of each conjugate pair of nuclei 

 in each basal cell of the spore-bed of every aecidium which the 

 mycelium produces. The details of the process by which the two 

 kinds of nuclei come to be assembled as conjugate pairs in the 

 basal cells may possibly vary in different species, in which case the 

 final part of the hypothesis might require modification. 



Assuming that, when two haploid mycelia (A) and (a) of the 

 same Rust species meet, they unite and that, sub.sequently, {A) 

 nuclei move through the (a) mycelium and {a) nuclei through the 

 (A) mycelium, it is clear (1) that the septa must break down 

 jiartially or wholly (as in Hymenc mycetes) to permit of the progress 

 of the nuclei and (2) that two nuclei of opposite sex, when they 

 meet in one and the same mycelium, are inhibited from fusing 

 with one another. The assumption of such an inhibition accounts 

 for the fact that the nuclei of opposite sex assembled in the basal 

 cells of each aecidium as conjugate nuclei do not fuse with one 

 another but divide conjugately. 



Assuming that the essential features of the diploidisation 

 process as it goes on in a multicellular and multinuclear haploid 

 mycelium of a Rust Fungus has been correctly indicated in Hypo- 

 thesis No. 2, we are justified in concluding that the diploidisation 

 process in the Uredineae, just as in the Hymenomycetes, is dependent 

 on the avoidance of the formation of (2n) nuclei. In the Uredineae 

 this results in the free association of nuclei of one sex with those of 

 the other sex in the mycelium becoming diploidised and, finally, in 



