2 BOTANICAL GAZETTE [july 



The cytological studies presented here will be confined chiefly to 

 the phenomena of synapsis and reduction in the pollen mother cell. 

 Various forms have been studied, a complete series of stages being 

 obtained in some forms and a partial series or only a few stages being 

 examined in others. The forms investigated include (i) O. rubri- 

 nervis, (2) O. Lamarckiana, (3) O. gigas, (4) O. nanella, (5) O. biennis 

 cruciata, a variety of the European O. biennis, (6) both O. lata (see 12) 

 and O. Lamarckiana from the F, of O. lata-XO. Lamarckiana, and (7) 

 plants resembling O. gigas, from the F, of O. lataXO. gigas. Pre- 

 liminary reports have already been made upon some of these studies, 

 in various connections (11, 12, 13, 14, 15). Reference will be made 

 to some of these results later. 



The material from each individual was collected separately in 

 nearly all cases, in order to observe possible individual differences in 

 the same race, either in the number of chromosomes or in other 

 cytological features. I am indebted to Mr. C. H. Shattuck for 

 making a number of these collections. The material for the study of 

 O. rubrinervis was obtained from a number of individuals grown in 

 two different seasons and representing several strains derived from the 

 same original pedigree. Sections were cut from six of these, and it 

 may be stated here that in O. rubrinervis no individual differences 

 were discovered, either in the number of chromosomes, which was 

 14 in all cases, or in any other features. In some of the other mutants, 

 also, a number of individuals were examined. It was found necessary 

 to reserve the account of O. gigas, which presents several features of 

 special interest, for a separate paper. A preliminary report on this 

 form has already been made (14, 15). 



For various reasons, O. rubrinervis was chosen as the most favor- 

 able form for a thorough study of synapsis and reduction. The 

 nuclei and chromosomes of Oenothera are small, and for this reason 

 the selection of the most favorable type for study is a matter of some 

 importance. In O. rubrinervis the pollen mother cells, although 

 they vary much in size, are usually considerably larger than in 

 O. Lamarckiana, the nuclei being also proportionately larger. The 

 reason for this will be explained later. The chromosome number 

 being low in most of the forms (2^=14, tf =7), they can be counted 

 without any difficulty, notwithstanding their small size. Another 



