332 THE BASIDIOMYCETES 



related Cupressineae and their aecial stage on the Maleae, and that 

 such a large proportion of the species of Puccinia and Uromyces, 

 having teliospores on grasses and sedges, produce aecia on the 

 Compositae or Ranunculaceae. Facts like these stimulate inter- 

 est in the whole problem of heteroecism, and the conclusion 

 seems inevitable that there is as yet no known reason why rusts 

 are limited and grouped in their aecial and telial host relationships. 



Sexuality of the Uredinales 



jManv students of fungi have concerned themselves with the 

 sexuality of rusts. Some of the results of their work have al- 

 ready been given in this report, especially if they deal with the 

 relationship of pycniospores to the origin of the dicaryotic con- 

 dition in aeciospore mother cells [Craigie (1927, 1931)]. 



Presumably the pycniospores in many species of rusts function 

 as male cells, fusing with trichogynes that project to the surface 

 from basal cells in the developing aecia. It must be remembered, 

 however, that some rusts are not known to possess pycnia and 

 that the pairing of nuclei must therefore be accomplished by 

 some other mechanism. This subject cannot be seen in proper 

 perspective, however, unless a few of the other important re- 

 ports on the sexuality of rusts are given attention. Apparently 

 the work of Poirault and Raciborski (1895) in 1895 constitutes 

 the first real cytological contribution to knowledge of rusts. 

 They described conjugate nuclear division but were in error con- 

 cerning the chromosome number. The next year the extensive 

 study of Sappin-Trouffy (1896) appeared, describing and amply 

 illustrating soral development in 10 genera and 36 species of rusts. 

 He also determined that the paired nuclei fuse during matura- 

 tion of the teliospore and that reduction-division occurs early 

 in the course of promycelial formation. 



Interest in the sexuality of rusts was keenly stimulated soon 

 thereafter by Blackman (1904). From examination of the aecia 

 of Fhraginidhmt violaceinn he described the migration of a small 

 nucleus from an ordinary vegetative cell into a larger special cell 

 (female cell) containing a larger nucleus. This process was re- 

 garded as the initiation of the sexual act, and it was held that ac- 

 tual fusion was delayed until maturation of the teliospore. He 

 viewed the buffer cells above the chains of aeciospores as degener- 



