i 9 o8] GATES— REDUCTION IN OENOTHERA 29 



are external differences between the plants having 14 chromosomes 

 and those of the same race having 15, is as yet unknown. But it is 

 quite conceivable that no such differences will be found, for if the 

 sporophyte chromosomes consist of two complete sets (and for a 

 variety of reasons this seems the only tenable view at the present time 

 if we assume qualitative differences at all), the presence of an addi- 

 tional chromosome, which is already present in duplicate, would 

 scarcely be expected visibly to affect the plant. 



Rosenberg (26) has found an analogous situation in Hieracium. 

 For example, H. excellensXH. Pilosella gives hybrids with different 

 numbers of chromosomes. This he ascribes to the fact that the eggs 

 of H. excellens differ in their numbers of chromosomes, which he finds 

 is due to irregularities in chromosome distribution during the reduction 

 divisions. The writer has pointed out elsewhere (12) certain similari- 

 ties between the hybridization phenomena in Hieracium and Oeno- 

 thera, and this seems to be a further similarity between the two genera. 



Rosenberg (27) has since shown that H. excellens produces three 

 kinds of embryo sacs: (1) Normal embryo sacs which require fertili- 

 zation for their development. These are presumably the only ones 

 which can be hybridized. The egg cells in these sacs vary in their 

 number of chromosomes owing to the fact that some of the chromo- 

 somes, lacking in "affinity," remain univalent (that is, fail to pair) 

 during the heterotypic mitosis and are irregularly distributed. It is 

 evident that this lack of affinity between chromosomes is similar to that 

 in. Oenothera. (2) In rare cases apogamous embryo sacs are formed 

 after a single division of the megaspore mother cell, and without 

 reduction. (3) More frequently the condition occurs which Rosen- 

 berg calls apospory, in which tetrad formation takes place and then 

 an adjacent cell of the nucellus enlarges, displaces the tetrad, and 

 forms an embryo sac without reduction. 



Summary 



In conclusion a brief summary of the facts and considerations 

 here presented will be useful. 



1. In Oenothera the heterotypic mitosis is a reduction division, 

 separating whole chromosomes which lie successively on the spirem. 

 The homotypic mitosis is an equation division, separating the longi- 



