1915] 



ATKINSON PHYLOGENY IN THE ASCOMYCETES 345 



in mass. The process is thus a reversible one, and by a sort 

 of see-saw growth of nucleus and cytoplasm the ascus cell is 

 pushed up to the large size characteristic of spore mother 

 cells. 



It is very true that the * * regulative function is a reversible 

 one," that an active cell with a large amount of cytoplasm 

 demands a correlative amount of nuclear substance, that the 

 increase in one may result in the increase of the other. Also 

 it is very true that the ascus belongs to the category of spore 

 mother cells, which are characterized by relatively large nuclei 

 and cytoplasmic mass compared with most vegetative cells, but 

 this does not explain why, when ascus or spore mother cell 

 formation is about to take place, cell division does not occur 

 at a period when the food relation would permit the forma- 

 tion of young uninucleate asci if these nuclei are bivalent in 

 nature. The regulative functions accompanying growth and 

 maturity of such a young gonotokont would assure sufficient 

 size, sufficient food material, and the necessary equilibrium. 

 The fact that asci in different species and groups vary so 

 greatly in size shows this, and also that there is no general 

 standard of mass in relation to surface area which would 

 demand two nuclei at the origin of the ascus. 



In fact it is very clear, from the morphological processes 

 which take place in the tip of the ascogenous hyphae of most 

 of the forms studied, that cell division, or cell wall formation, 

 is more likely governed by the last division of the two nuclei 

 so that the cell walls are laid down between the daughter 

 nuclei. If the inclusion and fusion of two nuclei in the young 

 ascus were controlled entirely by nutritive and cyto-regula- 

 tive processes, why are not sister nuclei included? Surely the 

 purely cyto-regulative functions would be just as well satis- 

 fied. It appears that in rare cases sister nuclei may be in- 

 cluded in the ascus (Brown, W. H., '10, in Leotia chloro- 

 cephala). 



Of the four nuclei resulting from the two successive divi- 

 sions of the zygote nucleus in Spirogyra, Chmielewski ('90) 

 states that two fuse to form the nucleus of the single germling 

 which is usually formed in the Zygnemaceae. Harper inter- 



