72 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



the nuclear phenomena are yet to be determined with certainty, the 

 union of the male and the female pronuclei is nowhere followed by such 

 a growth. Fertilization is the signal for rapid nuclear division, fol- 

 lowed at once or later by cell division, except in cases where the fertil- 

 ized egg is to serve as a resting stage. Dangeard's own theory of sex- 

 uality, based on the character of the gametes in the lower green algae, 

 is entirely opposed to the assumption that the fusion in the developing 

 ascus is equivalent to an ordinary union of sexual pronuclei. On the 

 other hand, the growth phenomena of the ascus are just what would 

 be expected in a highly specialized spore mother cell. 



It is possible that the nuclear fusion in the ascus, arising wholly 

 as I have described above in connection with the maintenance of the 

 nucleo-cytoplasmic equilibrium in the large ascus cell, may still func- 

 tionally satisfy in some minor degree the requirements of a sexual 

 fusion in case, in the course of development, it should be brought about 

 that the nuclei which so combine arise from a widely separated nuclear 

 ancestry. I have discussed this possibility in connection with both the 

 Basidiomycetes and the rusts in former papers (400 and 46), and have 

 pointed out that it is possible, though not at all proven, that the fusions 

 in the basidia and teleutospores may in some degree have functionally 

 replaced a true sexual union of nuclei, though occurring at the close of 

 a sporophyte generation. This, of course, does not mean that in any 

 sense whatever basidia or teleutospores can be considered as morpho- 

 logically equivalent to oogonia, nor that the series of cells from which 

 they arise are morphologically a gametophore, as Dangeard maintains. 

 Blackman's and Christman's discoveries show beyond all question that 

 the morphological equivalents of the oogonia and antheridia of other 

 related fungi and algae are not the teleutospores formed at the end of 

 a long series of binucleated cells. In the conjugating cells at the base 

 of the rows of aecidiospores we find the real equivalents of the sexual 

 cells elsewhere. Blackman's fertile cell is doubtless a modified egg cell. 

 The other cell of the pair we must conclude is a new structure which 

 has taken on a sexual function, since the spermatia, which are doubtless 

 the equivalents of the male cells of other groups, are still in existence in 

 the rusts. This carries with it the further conclusion that the processes 

 in the teleutospore and basidium form a parallel to those found in the 

 spore mother cells of higher plants. 



Blackman on- this basis denies all sexual significance to the fusion 

 in the teleutospore and considers the binucleated cells of the uredo and 

 teleuto mycelia as entirely equivalent physiologically, as well as mor- 

 phologically, to the uninucleated cells of the sporophyte of the higher 



