3<d BOTANICAL GAZETTE [july 



tudinal halves of the daughter chromosomes of the heterotypic 

 mitosis. Whether an approximation of threads or a split in a single 

 thread occurs in synapsis was not determined with certainty from the 

 observations, but various considerations lead to the belief that in 

 Oenothera the doubling is due to a split which closes up later, rather 

 than to an approximation of separate spirems. 



2. The conclusion that the method of reduction probably differs 

 in different genera is based on two considerations: (i) the fact that 

 in most of the recent accounts of synapsis and reduction in plants 

 a side-by-side pairing of chromosomes from maternal and paternal 

 spirems is described, while in Oenothera the members of a pair are 

 arranged end to end on a single spirem; and (2) on differences in 

 chromosome distribution during reduction in certain hybrids of 

 Drosera and of Oenothera (see p. 25). If reduction took place in 

 the same manner in both genera, the chromosome distribution during 

 reduction in these hybrids with reference to the parental chromosome 

 numbers should be the same in both, but this is not the case. 



3. Pairing between the definitive chromosomes during diakinesis 

 and the prophase of the heterotypic mitosis does not always take place, 

 owing to a weak attraction between the chromosomes. This allows 

 irregularities of distribution in the heterotypic mitosis, so that both 

 (unpaired) chromosomes belonging to one pair will occasionally enter 

 the same daughter nucleus (see p. 26). Germ cells will thus arise, 

 from which both members of a given pair of chromosomes are 

 absent. 



4. If we assume qualitative differences between the chromosomes 

 or parts of them, various types would be expected to originate in this 

 manner, each of them lacking the ability to develop certain qualities 

 possessed by the parent form. On this view the mutations of Oeno- 

 thera Lamarckiana are an instance of a process of analysis by which 

 from the parent form arises a series of types, each lacking in certain 

 characters or capacities possessed by the parent. This hypothesis 

 would account for the absence of reversions among Oenothera 

 mutants, and perhaps also for some of the peculiarities of hybridiza- 

 tion in Oenothera. This matter will be considered at another time. 

 This explanation does not apply to all the mutants, however; for 

 example, O. gigas. 



