1908] 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. excellens x H. 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. 
I. 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- 
