1907] GATES— HYBRIDIZATION OF OENOTHERA MUTANTS 17 



V 



:erm 



those of the father alone are contained in another, it is possible that two such 

 germ-cells may unite when fertilization takes place between these hybrids. In 

 such a case a plant completely resembling one of the ancestral forms would 

 arise, for it would have been produced from a germ-plasm which contains idants 

 of this species only. 



w 



He concludeSj however, (on a theoretical basis only) that this 

 rarely happens. 



To obtain the Mendehan ratios in hybrids on the basis of the 

 purity of the germ-cells, it is necessary to assume that the mechanism 

 of the reduction mitoses regularly segregates the maternal and paternal 

 chromosomes, and that their distribution to the daughter cells in such 

 cases is not merely a matter of chance, or that a part merely of the- 

 chromosomes from one parent can determine the dominance of that 

 form. Rosenberg''^ has elaborated such a scheme of chromosome 

 segregation in connection with his study of Drosera rotundijoliaX 

 D. longi folia. He found that in the metaphase of the heterotypic 

 mitosis in the pollen mother cell the chromosomes of the two parents, 



which can be dit^tinofnii^hpd bv fh^ir ciiV.p are not all oriented, with 



em 



from 



daughter nuclei, their orientation on the spindle being thus determined 

 by chance. The Drosera hybrid does not show complete dominance, 

 hut is intermediate, some of the characters being nearer to cither 

 parent. The character of the hybrid in later generations has not yet 



determined 



Mendcli 



that where the parents differ in one unit character this character will 

 be represented in the hybrid by a single Anlage from each parent. If 

 these Anlagen are assumed to be each represented by a single chromo- 

 some, the corresponding members of the other chromosome pairs being 

 qualitatively aHke, this would give, as Rosenberg'^ points out, a 



cnromosome basis for Mendelian segregation of characters. In the 

 formation of the megaspores and microspores, half would contain 

 the chromosome representing the peculiar paternal character, and 



'3 Rosenberg, O., Ueber Reduktionstheilung in Drosera. Meddel. Bot. Inst. 

 Stockholm, pp. 13. fig,_ 20. 1904. 



"* DeVries, HuGo.lBefruchtung und Ba.stardierung. Leipzig, pp. 62. 1903. 



