RUSTS 



If 



celium may eventually form a spore-forming structure, the pycniura, 

 on the upper surface of the barberry leaf (Fig. 7). In this structure 

 stalks of mycelium cut off uninucleate spores, the pycniospores 

 (spermatia, pycnidiospores). These are accompanied by a secretion 

 which attracts insects, the latter serving to transfer the pycniospores 

 to new barberry plants which, as will be shown, they cannot infect 

 directly. The sporidium is uninucleate and the mycelium and the 

 pycniospores derived from it are haploid. P. graminis is heterothallic. 



Fig. 8. Section through a "cluster cup" on the under surface of a barberry leaf 

 infected with Puccinia graminis showing the formation of aeciospores. 



If two sporidia of opposite mating type infect the same barberry 

 leaf near enough to each other to enable their mycelia to fuse, 

 "cluster cups" or aecia form on the under side of the barberry leaf. 

 The chains of aeciospores (aecidiospores) borne in these cluster cups 

 are binucleate. As with the mushrooms and the smuts, cellular fusion 

 without nuclear fusion has taken place. If only one sporidium falls 

 on a barberry leaf the haploid mycelium produces only pycnia and 

 pycniospores. The pycniospores themselves cannot infect the bar- 

 berry, but if pycniospores of mating type opposite to that of the 

 haploid mycelium already growing in a barberry leaf are carried to 

 that leaf in an infected area, these pycniospores fuse with the hap- 

 loid receptive hyphae protruding from the pycnia and their paired 

 nuclei are thought to migrate through the mycelium. Aecia and bi- 

 nucleate aeciospores (Fig. 8) are known to form as a result of such 

 a union of pycniospores and receptive hyphae. 



