W. Batbson and R. C. Punnett 299 



The recent work of de Vries' on Oenothera biennis and muricata has 

 provided other instances of dissimilarity between the factors borne by 

 the male and female organs of the same flower. In all these examples 

 it is almost certain that segregation cannot take place later than the 

 formation of the rudiments of the carpels and of the stamens respec- 

 tively. The only visible alternative is that in each sex the missing allelo- 

 morphs are represented by somatic parts of the sexual apparatus, which 

 for various reasons seems improbable. There is therefore much reason 

 for thinking that segregation can occur before gametogenesis begins, 

 but there is no indication as to which are the critical divisions. 



Now that we may regard the formation of four cells of composition 

 AB, Ab, aB, ab, as the foundation both of the coupling- and of the 

 repulsion-series the problem is manifestly somewhat simplified. The 

 time, excluding gametogenesis, at which we can most readily imagine 

 four such definite quadrants to be formed is during the delimitation of 

 the embryonic tissues. It is then that the plant is most clearly a 

 single geometrical system. Moreover the excess of gametes of parental 

 composition characterising the coupling- and repulsion-series must 

 certainly mean that the position of the planes of division by which 

 the four quadrants are constituted is determined with regard to the 

 gametes taking part in fertilisation. Though the relative positions of 

 the constituents of the cells may perhaps be maintained throughout the 

 history of the tissues, it is easier to suppose that the original planes of 

 embryonic division are determined according to those positions than 

 that their influence can operate after complex somatic differentiation 

 has been brought about. 



At some early stage in the embryonic development or perhaps in 

 later apical divisions we can suppose that the ?i — 1 cells of the parental 

 constitution are formed by successive periclinal and anticlinal divisions 

 of the original quadrants which occupy corresponding positions. The 

 accompanying diagram gives a schematic representation of the process 

 as we imagine it. Obviously it does not pretend to give more than 

 a logical or symbolic presentation of the phenomena. If such a system 

 of segregation is actually formed at the apex, it must be supposed that 

 the axes of the system revolve with the generating spiral. Whatever 

 hypothesis be assumed the following points remain for consideration. 



1. We are as yet unable to imagine any simple system by which 

 the four original quadrants can be formed by two similar divisions. 

 Evidently there must be two cell-divisions, and if in one of them we 



1 Biol. Centralbl. xxxi. 1911, p. 97. 



