286 RESEARCHES ON FUNGI 



the association of the nuclei in conjugate pairs in the basal cells 

 of the spore-bed of each aecidium. 



The diploidisation process in a haploid mycelium of a Rust 

 Fungus such as Puccinia graminis and P. helianthi can be initiated 

 not only (1) by another mycelium of opposite sex but, also, as 

 Craigie ^ has shown, (2) by means of pycnidiospores applied to the 

 mouths of pycnidia produced on the haploid mycehum ; but, 

 however it originates, its essential features are doubtless always 

 the same. Possibly, as Miss Allen's work on Puccinia graminis 

 suggests,^ the pycnidiospores put out germ-tubes which fuse with 

 the paraphyses (ostiolar hairs of a pycnidium) or other cells of the 

 mycelium already in the leaf and deliver to this mycelium nuclei 

 which travel down through the hyphae, divide, and find their way 

 to the spore-bed of each rudimentary aecidium where, by further 

 division and migration, they diploidise every cell that is destined 

 to produce a chain of aecidiospores.^ 



1 J. H. Craigie, " Discovery of the Function of the Pycnia of the Rust Fungi," 

 Nature, November, 1927. 



2 Ruth F. Allen, " A Cytological Study of Heterothallism in Puccinia graminis,'' 

 Journ. Agric. Research, vol. xl, 1930, pp. 585-614. 



3 It is possible that in the Uredineae, just as in the Hymenomycetes, there are 

 included not only heterothallic species, but also homothallic. Among the Rust 

 species which I suspect are homothallic are short-cycle species devoid of pycnidia, 

 e.g. Puccinia Malvacearum. This species is entirely dependent upon sporidia for 

 its dissemination. In a garden at Kew, in the summer of 1930, on searching the 

 Hollyhocks {Althaea rosea), I found that they were entirely free from P. Malva- 

 cearum, except for one leaf on which was a solitary pustule bearing the usual 

 chocolate- brown teleutospores. It seems unlikely that this solitary pustule should 

 have arisen from two sporidia of opposite sex which happened to have been 

 blown into the garden and to have settled on the leaf in exactly the same spot, 

 and the pustule could not have arisen from a single haploid sporidium and then 

 have been diploidised by pycnidiospores brought by insects from a distance, for 

 P. Malvacearum has no pycnidia. In the heterothallic Copiini oidia are generally 

 present on the haploid mycelia, but in the homothallic species, Coprinus stercorarius, 

 C. sterquilinus, and C. narcoticus, they are absent. From the investigations made 

 by H. J. Brodie, working under my direction (" The Oidia of Coprinus lagopus and 

 their Relation with Insects"; to appear in the Anrnils of Botany, April, 1931), 

 there is every reason to believe that the oidia of the heterothallic Coprini and the 

 pycnidiospores of the heterothallic Uredineae are comparable in their diploidising 

 function. In a homothallic Coprinus oidia would be useless, and in a homothallic 

 Puccinia pycnidiospores would be equally useless. Perhaps it is on account of 

 the early setting in of the diploid phase in mycelia of monobasidiosporous origin 

 that oidia and pycnidiospores have ceased to be developed in homothallic Coprini 

 and the presumably homothallic Puccinia Malvacearum respectively. 



