64 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



The problem is not more difficult, perhaps, in the case of the fusion 

 in the ascus than in that of the fusion of the polar nuclei and the second 

 sperm nucleus in the embryo-sac, or the second nuclear fusion described 

 by Chmielewski ( 14) as taking place in the formation of the zygospore 

 in Spirogyra, or the fusion vegatative nuclei in endosperm cells (87), 

 or the experimentally induced nuclear fusions observed by Nemec (70) 

 in root cells. It must be admitted that there has been so far no general 

 agreement as to either the morphological or physiological significance 

 of any of these processes, and doubtless we need more facts as to the 

 relation of the nucleus to other processes in the cell besides fertilization 

 before a final solution can be reached. It is a question, further, how 

 much any two of the individual cases mentioned have in common. 

 Each may well be, to a considerable extent, the reaction of the nuclei 

 to different conditions and with quite different results for the cell. 

 With the exception of the second fusion in the zygospore of Spirogyra 

 they have, however, one very striking point in common. They all occur 

 in cells whose dimensions are, or become, greater than those of the ordi- 

 nary cells with which they are associated. This is strikingly true of the 

 ascus, which, as I have pointed out above, is gigantic in size compared 

 with any other cells of the mildew. Into it are poured, for the forma- 

 tion and nutrition of the spores, all the surplus food materials accumu- 

 lated in the vegetative cycle of the fungus. The injection of this 

 immense amount of food material into the cytoplasm and its consequent 

 rapid growth leads naturally to the expectation that a correspondingly 

 large nucleus must be formed ; and, as I have pointed out above, the 

 nucleus of these ascus cells is actually as much greater than the ordinary 

 vegetative nuclei as the ascus is larger than the vegetative cells. 



My studies of the nuclear processes in the ascus have from the 

 first led me to the conclusion that the nuclear fusion in the young ascus 

 was correlated in some way with the vegetative development of the rela- 

 tively gigantic size of the ascus as compared with other cells of the fun- 

 gus ; and in these facts of relative size I am convinced we have a basis 

 for correlating these cases of nuclear fusions with the broader facts as 

 to the relation of nuclear dimensions to cell dimensions which have been 

 frequently noted and recently have been given striking experimental 

 demonstration by Gerassimoff, R. Hertwig, and Boveri. The fact that 

 large cells in general have large or numerous nuclei and that small cells 

 have small or few nuclei is well known, but that this relation is funda- 

 mental and necessary was first shown by the experimenters just named. 



We may note, first, Gerassimoff's (26-29) results. As is well 

 known, by ingenious experimental methods cooling while cell division 



