NUCLEAR FUSION IN THE ASCUS. 69 



may be determined by minor factors in each case, whether the nucleo- 

 cytoplasmic equilibrium be established with or without nuclear fusion. 



Nemec (70) has pointed out that the fusion of nuclei, far from 

 always having a sexual significance, may well be considered in many 

 cases merely as a consequence of the inclusion of two nuclei in the 

 same cytoplasmic system, and that the maintenance of many nuclei in 

 the large vegetative cells of the algae and fungi may be for the purpose 

 of distributing the nuclear material through the cell, so that its relation 

 to growth and metabolism may be more perfectly and readily main- 

 tained a view which appears to be generally accepted by the students 

 of coenocytic cells. On the basis of his own experiments he further 

 contends that the cell fusion and not the nuclear fusion is the essential 

 feature in sexual reproduction, and that the fusion of the pronuclei also 

 may be regarded as in the nature- of a necessary sequence of the cell 

 fusion without thereby detracting in any degree from the important 

 physiological significance of the former. Blackman's (8) and Christ- 

 man's (15) discoveries in the aecidium, discussed above, support this 

 view unequivocally. In the light of these results we are bound to con- 

 clude that in the rusts the process of nuclear fusion is associated with 

 the process of chromosome reduction rather than with fertilization. 



Nemec has further observed (70) that these non-sexual fusions in the 

 root cells result in doubling the chromosome number, which, however, 

 is later apparently reduced again to the normal number for the sporo- 

 phyte. This reduction, he concludes, is an autoregulative function. 

 In the ascus the nuclear fusion, as I have described above, results in an 

 immediate apparent reduction of the chromosome number. That the 

 reduction is immediate in the ascus and occurs somewhat less promptly 

 in the root cells may well be due to the fact that the whole process in 

 the latter demands new adjustments, while in the ascus it is normally 

 recurrent at a definite point in the life cycle and may well have been 

 perfected in its operation by selection. 



The nucleus of the ascus under normal conditions, since, as we 

 shall see further on, it presumptively must contain quadrivalent chromo- 

 somes without thereby having its own apparent number increased, must 

 be considered to be in a sense hypertrophied as to its chromatin content 

 when compared with the ordinary nuclei of the mildew. As noted also, 

 the whole fungus pours its excess of nutriment into the ascus, and both 

 nucleus and cytoplasm increase greatly in size. The condition is pos- 

 sibly parallel to that of the nuclei in Hertwig's (44) cultures of Actinos- 

 phaerium, which he kept for long periods under conditions of over- 

 feeding. In such cases he was able to observe that the hypertrophied 



