ALTERNATION OF GENERATIONS. 87 



thus attempt to systematize our very fragmentary knowledge of the 

 higher thallophytes ; and I am very far from believing that such specu- 

 lations will be so fortunate as to achieve a confirmation in all respects. 

 Such observations as Blackman's and Christman's on the aecidium and 

 Mottier's on the tetrasporange are sufficient to indicate how far from 

 complete our knowledge is, even on some of the most commonly studied 

 structures and processes in these plants. My conceptions as to alterna- 

 tion of generations in the higher fungi may be briefly stated as follows : 



In the rusts we have sexual reproduction by vegetative fertiliza- 

 tion. The fusing cells are perhaps morphologically vegetative offshoots 

 of an egg-cell. Alternation of generations is present and the sporo- 

 phyte generation is modified by the development of conjugate nuclear 

 division through its whole cycle, resulting in a failure of the sexual pro- 

 nuclei to fuse at the time of fertilization. Conjugate division may have 

 arisen here, as it is perhaps beginning in Pyronema, immediately prior 

 to a nuclear fusion in the spore-mother cell, and have worked backward 

 through the sporophyte, thus tending to give more and more of a func- 

 tional and sexual significance to the fusion in the spore mother cell, 

 until, finally reaching the fertilized egg, the fusion of the pronuclei 

 disappeared. 



In the Basidiomyctes by apogamy sexual cell fusion may have dis- 

 appeared or we may have vegetative fertilization. The sporophyte cells 

 arise as ordinary hyphal cells which become binucleated, and conjugate 

 nuclear division extends through the entire sporophyte generation. 



In the Ascomycetes we have sexual reproduction and alternation 

 of generations, modified by the adaptation of the spore mother cell as 

 an explosive organ for the dissemination of the spores and as a storage 

 reservoir for the production of resting spores with a large supply of 

 metaplasmic reserve products. The development of the relatively 

 gigantic size of the ascus led, on the principle of the nucleo-cytoplasmic 

 relation, to the increase of its nuclear content by inhibition of cell 

 division and to the fusion of sporophyte nuclei ; and this in turn neces- 

 sitated a triple instead of a double division in the reduction of the 

 chromosome number. 



MADISON, WISCONSIN, April, 1905. 



