SIGNIFICANCE OF CONJUGATE NUCLEI 281 



whenever such a division is able to promote the diploidisation of a 

 haploid mycelium. 



The Uredineae. The Rust Fungi, a group of highly speciahsed 

 parasites, have probably been evolved from the Hymenomycetes 

 which they resemble : (1) in producing basidia and basidiospores, 

 (2) in the mode of development and discharge of their basidio- 

 spores, (3) in the absence of sexual organs and gametes, and (4) in 

 the general nature of their sexual process. 



The sexual process in a heterothallic Rust Fungus, such as 

 Puccinia graminis or P. helianthi, when initiated by two haploid 

 mycelia of opposite sex derived from two basidiospores of opposite 

 sex, resembles that of the Hymenomycetes already described ; for 

 it takes place as follows. Two haploid mycelia of opposite sex 

 derived from two basidiospores (sporidia) of opposite sex meet 

 within a host-leaf, unite with one another, and mutually diploidise 

 one another so that, in the spore-bed of each aecidium, there are 

 produced basal cells each of which contains a pair of conjugate 

 nuclei.^ As soon as the diploidisation process has been completed, 

 cell-division preceded by conjugate nuclear division sets in and 

 continues during the production of the aecidiospores, the develop- 

 ment of the mycelium derived from an aecidiospore, the production 

 of uredospores, the development of the mycelium derived from a 

 uredospore, and the production of teleutospores. As soon as the 

 teleutospores have been formed, conjugate nuclear division comes 

 to an end and the two nuclei of the conjugate pair in each cell of 

 each teleutospore fuse together. In the basidium (promycelium) 

 developed from each cell of a teleutospore the fusion nucleus under- 

 goes two successive divisions accompanied by a reduction in the 

 number of chromosomes to one-half and a segregation of the sex 

 and other heritable factors. Finally, the basidium gives rise to 

 four sterigmata and four spores and each of the four {n) nuclei 

 creeps through a sterigma and so enters a spore. Thus each 

 basidiospore is a haploid cell. 



If, as we have seen, conjugate nuclei are essential to the 

 working of the diploidisation process as it occurs in a haploid 

 mycelium of one of the Hymenomycetes, then, in all probabihty, 

 1 J. H. Craigie, " Experiments on Sex in Rust Fungi," Nature, July, 1927. 



