542 



THE AMERICAN NATURALIST [Vol. XLIV 



gans and their function as spore mother-cells which are 

 to undergo reduction divisions and close the sporophyte 

 generation. 



From this standpoint it is to be remembered that we 

 have considerable evidence that in the rusts with abbre- 

 viated life cycles the binucleated condition and con- 

 jugate division extend back from the endokaryogamy to 

 more or less vague and uncertain points in the life cycle 

 of the sporophyte. For example, in the short-cycled rusts 

 with only teleutospores Blackman, Maire and Olive find 

 that the binucleated condition extends back from the 

 teleutospore to some undetermined point in the mycelium 

 from which the teleutospore sorus arises. As already 

 noted, it is certain that in various basidiomycetes the 

 binucleated condition extends back through the carpo- 

 phore to some not sharply marked stage in the mycelium. 

 Similar conditions may be present in the smuts. 



In Pyronema I have found that the paired nuclei in the 

 young ascus arise by simultaneous division of two mother 

 nuclei in the tip of the ascogenous hypha from which an 

 ascus is to arise and that the spindles are so placed that 

 the two nuclei which are cut off in the young ascus and 

 subsequently fuse are not sister nuclei. This final 

 nuclear division is similar to the conjugate nuclear divi- 

 sions in the rusts. Kecently Guilliermond and Maire 

 have described cases among certain of the larger dis- 

 comycetes in which the ascogenous hyphae were found to 

 consist of binucleated cells for some distance below the 

 young ascus, and suggest that these are comparable to 

 the binucleated series of cells in the rusts. In the major- 

 ity of cases so far described the ascogenous hyphae are 

 multinucleated as in Pyronema and Aseobolus. In the 

 mildews the cells of the ascogon below the ascus are uni- 

 nucleated and it does not appear that the nuclei which 

 fuse in the ascus come from any such distinct lines of 

 nuclear descent as do those which fuse in the teleutospore 

 of the rusts. 



The binucleated condition may be working backward 



