282 RESEARCHES ON FUNGI 



it is essential to the working of the diploid isation process as it 

 occurs in a haploid mycelium of a Rust Fungus. 



From the foregoing it is clear that, in the Uredineae, during 

 the diploidisation process, the nuclei of opposite sex do not unite 

 with one another to form {2n) nuclei, but retain their identity and 

 eventually become associated with one another as conjugate nuclei 

 (,j)_)_(7i) in the basal cells of the spore-bed of each young aecidium. 



Before it is possible to decide what biological advantage, if any, 

 accrues to the Uredineae by the avoidance of the formation of {2n) 

 nuclei and the eventual arrangement of the nuclei in conjugate 

 pairs {n)-]-{n) during the diploidisation process, it is necessary to 

 form some definite conception of the precise way in which the 

 diploidisation process is effected ; and this will now be attempted. 



Let us suppose that two basidiospores (sporidia) of opposite 

 sex, {A) and {a), of some heterothalHc long-cycle Rust Fungus 

 have settled on a host-leaf near one another and have germinated, 

 so that two young haploid mycelia of opposite sex are present in 

 the leaf. Each haploid mycelium forms a rust pustule. As 

 growth continues, the two myceha come into contact with one 

 another and the two rust pustules coalesce. How do these two 

 mycelia, on coming into contact with one another, interact so as 

 to form conjugate nuclei in the basal cells of the aecidia ? In 

 other words : how does the diploidisation process work ? There 

 are various possible answers to this question and two of them will 

 now be brought forward and discussed. 



Hypothesis No. 1. The two haploid myceUa (A) and (a), as 

 their growth proceeds, intermingle so that, eventually, two kinds 

 of hyphae [A) and (a) come to be arranged almost alternately in 

 the spore-bed of each young aecidium. Then, in the spore-bed, 

 pairs of {A) and {a) cells combine to form conjugate nuclei : either, 

 as in Phragmidium violaceum ^ or Puccinia poarum 2 by a cell- wall 

 becoming perforated to allow of an (A) nucleus migrating into an 

 (a) cell or vice versa, or, as in a number of other rust species, by 



1 V. H. Blackman, " On the Fertilization, Alternation of Generations, and 

 General Cytology of the Uredineae," Annals of Botany, vol. xviii, 1904, pp. 323-373. 



2 V. H. Blackman and H. C. I. Eraser, " Further Studies on the Sexuality of 

 the Uredineae," Annals of Botany, vol. xx, 1906, pp. 35-48. 



