i 9 o8] GATES— REDUCTION IN OENOTHERA 3 



notable advantage in comparing this with other studies in reduction 

 is in the shape of the chromosomes, which are globular or somewhat 

 oblong or cylindrical in most stages of mitosis, and are never greatly 

 elongated or looped. For this reason it is a comparatively easy 

 matter to obtain accurate counts of the chromosomes in the pollen 

 mother cells of any of the forms. This shape is also found to be very 

 advantageous in a study of the events of reduction following synapsis. 

 The appearances are clear and easily interpreted, in striking contrast 

 to the forms with long twisted chromosomes, such as have been made 

 the basis of many of the studies on reduction in plants. 



On the other hand, the somatic nuclei and chromosomes are very 

 much smaller, and in metaphase the latter are elongated and looped, 

 making it impossible to count them with the same degree of accuracy. 

 Some of these appearances have already been described elsewhere 

 (12, p. 19). Thus while it was found that the chromosomes could 

 be counted almost equally well in pollen mother cells of all the 

 forms studied, O. rubrinervis was found to be especially favorable 

 for the investigation of reduction phenomena, especially the events 

 of synapsis and the prophases of the heterotypic mitosis. The 

 account given here will refer throughout to O. rubrinervis, with 

 occasional comparisons with other forms. Later papers will take 

 up these other forms in detail, in so far as this is necessary after the 

 account presented here. Special attention will be given at that time, 

 in particular, to the later stages, beginning with the telophase of 

 the heterotypic mitosis, and also to the interesting conditions in some 

 of the hybrids. The detailed account in O. rubrinervis will not be 

 carried farther than the metaphase of the heterotypic mitosis, at 

 which time the essential events have already taken place. 



Methods 



The usual methods of cytological technique were employed, 

 various chrom-acetic and chrom-osmo-acetic solutions being tried 

 until satisfactory fixation was obtained. The thickness of the sec- 

 tions varied from 4 to 10 p. The latter thickness was found most 

 favorable for counting chromosomes, because it is somewhat greater 

 than the diameter of the nuclei, many of which in such sections were 

 therefore uncut. It is possible to determine easily whether a nucleus 

 has been cut by the knife by examining it in low and high focus. The 



