62 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



as 8 chromosomes in the equatorial plate a little later. Doubtless in 

 the lower algae and fungi cases of sexual fusion, followed by immediate 

 reduction, may exist, but not in connection with any such elaborate 

 differentiation of fruit-forms as we find in the conidia and ascocarps 

 of the Ascomycetes. 



In the oogonium of the mildew, on the other hand, the fusion of 

 sexual nuclei from separate gametes is not followed by any evidence of 

 reduction processes, but results in a vegetative growth plainly com- 

 parable to the development of a sporophyte generation, as has so many 

 times been suggested by the older authors. I have not been able so 

 far to count the chromosomes in the fertilized egg-nucleus, but in the 

 growth of the ascogonium and ascogenous hyphas before the ascus is 

 formed there is no evidence of the existence of any synapsis stage or 

 special double division, such as is now universally recognized as asso- 

 ciated with the process of chromosome reduction. We shall see also 

 that there is evidence for believing that the chromosomes of the nuclei 

 which fuse in the ascus are already bivalent structures as a result of 

 the previous nuclear fusion in the oogonium. 



We must note, as bearing on this point, that while the process of 

 fertilization, in all cases where it has been thoroughly investigated, is 

 the formation of a cell with the double number of chromosomes, the 

 combination of these chromosomes in the single nucleus may be either 

 immediate or a more gradual process, as is conspicuous in the embryos 

 of Cyclops; and Blackman's (8) and Christman's (15) interesting dis- 

 coveries in the aecidium of the rust show that the final combination of 

 the nuclei may be delayed through an indefinite number of cell genera- 

 tions. The ultimate fusion in this case seems to be associated with the 

 process of chromosome reduction and the development of spores. 



Blackman's discovery of his so-called vegetative fertilization in the 

 aecidium of the rusts, bearing as it does on the whole question of the 

 homologies and relationships of the higher fungi, as well as on the 

 general question of fertilization and alternation of generations, is cer- 

 tainly of the highest importance. With his account and that of Christ- 

 man we may believe that the historic question as to the sexuality of the 

 secidium is finally settled. Blackman finds that the basal cell of each 

 row of aecidiospores in Phragmidium violaceum is fertilized by the 

 migration through its wall of a nucleus from a neighboring cell. The 

 pronuclei do not fuse, but divide by conjugate division, and thus the 

 cycle of binucleated cells which terminates with the nuclear fusion in the 

 teleutospore is begun. There is no wide communicating pore between 

 the gametes, as found by Blackman. The nucleus passes through the 



