HOMOTHALL1SM AND HETEROTHALLISM 333 



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differ either widely or slightly in every observable character or 

 combination of characters. New ones are continually being pro- 

 duced as a result of mutation and of recombinations resulting 

 from interbiotypic hybridization." 



In Uredinales. The Uredinales, or rust fungi, are a group of 

 obligate parasites of enormous economic importance, because 

 many of them attack crop plants. Although many studies, from 

 which have come a number of fundamentals concepts, have been 

 concerned with their sexuality, an understanding of this subject 

 was first established by the investigations of Craigie (1927, 1927a, 

 1928). He showed that the pycnia are functional structures and 

 that the pycniospores are essential in diploidization. 



Undoubtedly many rusts are heterothallic, for Craigie's studies 

 have shown that such is the situation in Pucclma graminis, P. 

 helianthi, P. coronata, P. pringsheimiana, and Gymno sporangium 

 sp. At germination of the teliospore, whose mature cells are uni- 

 nucleate, the nucleus divides meiotically within the promycelium, 

 the homolomje of the basidium, and the four resultant nuclei are 

 haploid. Each migrates through a sterigma into the basidiospore 

 that arises at the apex of a sterigma. Craigie found that these 

 basidiospores are of either (-+-) or ( — ) potentialities. If mono- 

 sporidial inoculations are made, pycnia containing pycniospores 

 are developed. However, aecia never develop in association with 

 such pycnia unless pycniospores from a pycnium of opposite sex 

 are applied. In nature this interchange of pycniospores is accom- 

 plished either by insects attracted to the sugary exudate in which 

 pycniospores are embedded or by water. Buller (1940) has 

 shown that flexuous hyphae extend from the orifices of the pycnia 

 and that pycniospores fuse with these flexuous hyphae. The 

 pycniospores are thus spermatia, and the flexuous hyphae are re- 

 ceptive surfaces comparable to trichogynes. Buller (1940) has 

 observed flexuous hyphae in 21 species belonging in Coleospo- 

 rium, Cronartium, Gymnoconia, Gymnosporangium, Melampsora, 

 Alelampsorella, Alilesia, Phragmidium, Pucciniastrum, Puccinia, 

 and Uromyces. 



If spermatization is accomplished, aecia bearing dicaryotic 

 aeciospores with conjugate, (n) -f (72), nuclei are developed. 



In full-cycled rusts, not only the aeciospores but also the myce- 

 lia arising when they germinate, the urediniospores and the my- 

 celia arising from their germination, and the young teliospores 



