K. — BOTANY. 



183 



process in order lo explain the phenomenon of partial hnkage is, it 

 will be seen, inherent alike in both theories. When, however, we come 

 to examine the general applicability of Morgan's theory we are con- 

 fronted with a considerable body of facts among plants which we find 

 difficult to reconcile with the requirement that factorial segregation is 

 accomplished by means of the reduction division. An instance in 

 which this is particularly clearly indicated is that of the sulphur-white 

 Stock. I have chosen this example because here we have to do with 

 two characters which are distinguished with the utmost sharpness, 

 viz. : plastid colour and flower form. The peculiar behaviour of this 

 strain is due to the fact that not only are the two factors for flower form 

 (singleness and doubleness) differently distributed to the male and 

 female sides of the individual, as in all double-throwing Stocks, but the 

 factor controlHng plastid colour likewise shows linkage with the sex 

 nature of the germ cells. As a result every individual, even though 

 self- fertilised, yields a mixed offspring, consisting chiefly of single 

 whites and double creams, but including a small percentage of double 

 whites. So far as the ovules are concerned, the mode of inheritance can 

 be accounted for on either theory. According to the reduplication 

 hypothesis the factors X Y '^ producing singleness and W giving white 

 plastids are partially coupled so as to give the gametic ratio on the 

 female side 7WXY: IWXy: 1 wxY : 7wxy.^* On the chromosome 

 scheme the factorial group WXY must be assumed to be disposed in 

 one member of the bivalent chromosome formed at. meiosis, the corre- 

 sponding recessive allelomorphs wxy in the other. If the three factors 

 be supposed to be arranged in the chromosome in alphabetical order, 

 and if, on separation, a break takes place between the loci of the two 

 factors for flower form (as shown), so as to give ' cross-overs ' of Y 



w 



>^ 



w 



X 



^ 



Ovules 



Pollen, 



" The letters X and Y are used here to denote particular factors, not, as in 

 Morgan's scheme, the entire sex-chromosomes. 

 " Or possibly 15 : 1 : 1 : 15. '^ >> '■' 



