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SCIENCE 



[N. 8. Vol. XXXIX. No. lOOO 



very base of the chain of secidiospores. 

 Maire (1900) first stated clearly the whole 

 nuclear cycle in rusts : Beginning with the 

 binueleate fficidiospore there follows, in the 

 wheat rust, e. g., the uredo, or rust stage 

 which has a binueleate mycelium and forms 

 binueleate uredospores for several genera- 

 tions. The two nuclei of the young teleuto- 

 spore, finally formed on this mycelium, fuse 

 as the spore matures. The two divisions of 

 this fusion nucleus in the promycelium give 

 rise to the four nuclei of the four sporidia 

 which germinate to the uninucleate cluster 

 cup mycelium on the barberry. Maire saw 

 in this life history a real alternation of 

 generations, the gametophyte or X genera- 

 tion beginning with the sporidium, the 

 sporophyte or 2X generation, with the 

 mother cell of the secidiospore chain. 



Blackman (1904) and Christman (1905) 

 discovered the origin of the binueleate con- 

 dition of this mother-cell in species of 

 Pliragmidium. It there arises by the migra- 

 tion of a nucleus from one cell into another, 

 or by the fusion of the cytoplasm of two 

 cells to form the mother-cell of the spore 

 chain. The two nuclei thus brought to- 

 gether divide simultaneously or conju- 

 gately, each contributing a nucleus to the 

 first and to each succeeding spore. This 

 conjugate division of the paired nuclei and 

 their descendants was shown to occur all 

 through the uredo generation up to the 

 formation of the young teleutospore. In 

 the interpretation of their discoveries 

 Blackman and Christman differ more 

 widely than in the facts reported. The 

 former reasserts the surmise of Meyen and 

 believes the basal cells of the spore chain 

 are oogonia which were primitively fertil- 

 ized by the now functionless spermatia, or 

 pycnospores, produced in separate organs 

 on the barberry leaf. Christman, on the 

 contrary, regards the fusing cells at the 

 base of the aecidium as the primitive, un- 



differentiated sexual organs of these fungi, 

 lie holds that male and female organs have 

 never become differentiated in this group, 

 and thinks that the spermatia are, or were, 

 propagative cells of the X generation. 



The observations of many workers on the 

 smuts and on the toadstools have shown the 

 frequent occurrence in them of an associa- 

 tion of nuclei and the final fusion of 

 two nuclei in the chlamydospore or the 

 basidium. The time and mode of asso- 

 ciation of the fusing nuclei, or of their 

 progenitors, are very different in different 

 forms. The fusion, and what appears to be 

 the reduction divisions are, however, con- 

 stant in location in each species, and are 

 always closely associated. Thus, e. g., the 

 nuclear fusion in the smuts often occurs in 

 the chlamydospore, according to Dangeard 

 (1893) and Rawitscher (1912), and reduc- 

 tion evidently follows immediately in the 

 next developmental phase, when this spore 

 germinates to form the sporidia. In the 

 toadstools, according to Wager (1893), 

 Dangeard (1894), Harper (1902), Nichols 

 (1904) and Levine (1913), the fusion of 

 nuclei occurs in the basidium, and the re- 

 duction at the very next division of this 

 fusion product, when the four spore nuclei 

 are formed. 



The striking uniformity with which the 

 apparent reduction occurs in all basidio- 

 mycetes, at the time of formation of the 

 sporidia or basidiospores, affords good evi- 

 dence that this type of spore-formation is 

 a long-established one, common to the whole 

 group. It thus supports Bref eld's view 

 that the promycelium of the smuts and 

 rusts is homologous with the basidium of 

 the higher forms. That the point in the 

 life cycle where the associated nuclei finally 

 fuse is the point at which it occurred in 

 the earliest basidiomycetes, is not so clear. 

 The modes of bringing about the first asso- 

 ciation of the paired nuclei are so varied that 



