SYNGAMY 231 



divide in unison before each cell division, their descendants eventually 

 fusing in the basidium. Here the nuclear fusion ("karyogamy") is 

 delayed until some time after cytoplasmic union ("plasmogamy"), these 

 two events being separated by a binucleate phase in the life cycle. 



In the rust fungi there are life cycles of several kinds, these differing 

 in the presence or absence of various spore types and of cell and nuclear 

 fusions. ^2 Some forms have uninucleate cells throughout the cycle, some 

 are entirely binucleate, and some have uninucleate and binucleate, or even 

 short plurinucleate, phases. In a number of forms, including Puccinia 

 graminis, the common wheat rust, each cell of the young teliospore is 

 binucleate. These two nuclei eventually fuse, and as the teliospore 

 germinates to form a promycelium, which is homologous with the basidium 

 of the hymenomycetes, the fusion nucleus divides to four which enter the 

 sporidia. Beginning with the work of Blackman (19046) and Christman 

 (1905) on Phragmidium and certain other genera, it was thought that the 

 binucleate condition originated by nuclear migration between cells or by 

 cell fusions, particularly in the basal region of the secial sorus. Evidence 

 recently brought forward ^^ indicates that in certain species of Puccinia 

 a mycelium developing from a single sporidium, or basidiospore, pro- 

 duces as a rule only sterile secia, and that chains of aeciospores develop 

 only when pycniospores of a different strain are added to the infec- 

 tion or when "plus" and "minus" mycelia develop close together. 

 Some of the cytological changes involved here have been ascertained. 

 Andrus (1931) has announced that in Uromyces appendiculatus the spore- 

 forming cells in the base of the secium are not formed by fusion but are 

 enlarged female cells which send elongated trichogyne-like hyphaj through 

 the opposite side of the leaf, where they meet and fuse with spermatia 

 (pycniospores) from infections of opposite strain. Nuclei apparently 

 pass through the septa of the trichogynes into the female cells, which 

 then proliferate and produce chains of binucleate seciospores. Similar in 

 many respects is R. F. Allen's (1932a) interpretation of the phenomena in 

 Puccinia triticina. Here the mycelium formed by a single sporidium 

 produces spermatia, receptive hyphee, and the primordia of aecia. After 

 nuclei from spermatia of the opposite strain enter the receptive hyphse, 

 the resulting binucleate (or plurinucleate) hyphae extend into young 

 secia and develop seciospores. 



In the smut fungi, as exemplified by species of Ustilago, the binucleate 

 condition arises as the result of a union of promycelial cells, sporidia, or 

 mycelial cells which they produce, the . two nuclei fusing later when 

 chlamydospores are differentiated. ^^ It is known further that the sporidia 



12 See Arthur (1929), Gaumann-C. Dodge (1928), B. O. Dodge (1929a), and 

 Jackson (1931). 



i^Craigie {I927ab, 1928, 1931), R. F. Allen (1929, 1930, 1932), Hanna (1929a). 

 1* P. A. Dangeard (1893), Lutman (1910), Rawitscher (1912), Kniep (1919, 1920). 



