74 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



such fixed point in the life cycle as in the rusts, and that in this respect 

 the Basidiomycetes represent a further stage in development following 

 the condition described in the rusts by Blackmail and Christman. There 

 are two possibilities as to the causes that have led to this further stage 

 in development. Either sexual reproduction by the union of differen- 

 tiated gamete cells is gradually disappearing under the influence of the 

 particular habits of life of the, fungi, the conditions in the higher Basidio- 

 mycetes representing a further stage in this process, or the new proc- 

 esses of conjugate division, followed by nuclear fusion, in connection 

 with chromosome reduction at the close of a sporophyte generation, has 

 tended to replace the earlier sexual fusion. Blackman is of the opinion 

 that such fertilizations by vegetative cells, as he finds in Phragmidium, 

 will also be found in the higher Basidiomycetes, but Miss Nichols has 

 made it plain that, for certain forms at least, no such assumption is at 

 all probable, and it is further possible that there are reduced forms 

 among the rusts in which the same condition prevails. 



On either of the above hypotheses such forms are possible, and if 

 the second hypothesis is accepted we have the key to the explanation 

 of the phenomena involved in the two nuclear fusions found in the 

 mildews, Pyronema, and presumably other Ascomycetes. I have pre- 

 sented above the evidence that nuclear fusion in the ascus arises in con- 

 nection with the processes involved in the maintenance of the nucleo- 

 cytoplasmic relation during the development of the relatively enormous 

 size of the ascus cell, and have pointed out that in its origin in this 

 fashion it has nothing to do with sexual reproduction in any respect. 

 If, however, we may assume that, with the development of a separate 

 ancestry for the fusing nuclei by simultaneous nuclear division, as found 

 in the bent-end cells of the ascogenous hyphse in Pyronema and, accord- 

 ing to Maire, in a series of binucleated ascogenous cells in Galactinia 

 and Acetabula (see also 31), the process has gained in some degree a 

 functional equivalence for sexual fusions, we can further assume that 

 this condition, perhaps working together with other influences such 

 as have led to parthenogensis in other fungi, has made possible the 

 occurrence of parthenogenesis or even apogamy in such Ascomycetes, 

 for example, as Pleospora and Teichospora. Such conjugate nuclear 

 division would originate, not as it apparently does at present in the 

 rusts through the failure of the pronuclei to combine in one, but in 

 connection with the development of the spore mother cell, as in Pyro- 

 nema. From this point we might expect the process to work back 

 in the ascogenous hyphse, as it appears to be doing in Galactinia and 

 Acetabula. Ultimately it might reach the egg-cells and result in the 



