8o SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



a reduction of the number of chromosomes is effected. The discov- 

 eries of Blackman, and even more convincingly those of Christman, by 

 proving that the binucleated cells of the rusts originate in an unques- 

 tionable fertilization, have put the existence of an alternation of genera- 

 tions in these forms beyond question. It is to be noted also what posi- 

 tive support the conditions in the rust give to the doctrine of the inde- 

 pendent persistence of the prochromosomes throughout the sporophyte 

 generation, which was beginning to be inferred from the phenomena in 

 the higher animals and plants. When the nuclei of the gametes persist 

 as independent structures up to the time of chromosome reduction in 

 the spore mother cell, there can be no question of the independent per- 

 sistence of the individual chromosomes from the two gametes. This 

 condition also enables us to establish beyond reasonable doubt the 

 existence of a stage in the life cycle with cells containing the double 

 chromosome number without an actual counting of the chromosomes. 

 It is, of course, highly desirable that the number of chromosomes in 

 the division in the teleutospore and elsewhere be established by actual 

 counting, but there can be no question, even without this further evi- 

 dence, that the main conclusions of these authors are fully justified. 

 The binucleated cells of the rust represent a stage with double chromo- 

 some number arising in a process of fertilization and closing with a 

 reduction of the chromosome number. A parallel with the alternation 

 of generations in the higher plants is thus fully established. 



If it could be further shown that while the antheridia and oogonia 

 unite and the male nucleus comes to lie in the egg beside the female 

 nucleus they still do not fuse, but maintain their independent existence 

 through the development of the ascogonium and ascogenous hyphse, 

 finally combining in the ascus, we should have in the mildews and Pyro- 

 nema an apparent parallel to the nuclear phenomena in the rust. Since, 

 however, the cells of the ascogonium in the mildews are uninucleated 

 and there is no evidence of any provision for maintaining separate 

 and parallel series of nuclei, such as are present in the rusts, it is plain 

 that even if we did not have a fusion of nuclei in the oogonium we 

 could assume no close parallelism between the mildews and rusts on 

 this basis. If we should assume also that in Pyronema the male and 

 female nuclei only become mingled but do not fuse in the oogonium, 

 there would seem to be equally little chance for maintaining any distinct 

 lines of nuclear descent in the multinucleated cells of the ascogenous 

 hyphas until at the very close of their development, when simultaneous 

 nuclear division occurs and provision is thus made that the nuclei of 

 the ascus shall at least not be sister nuclei. This single simtiltaneous 



