R. N. Salaman 31 



The ratio 9 : 7 is one very well-known in Mendelian analysis and is 

 evidence of the interaction of complementary factors belonging to 

 separate pairs of allelomorphs. 



Now if R be considered the factor which in presence of the 

 developer D converts the chromogen into a red pigment, then the 

 zygotic composition of " Flourball " should be written RrDd, which 

 will on selfing give plants with the following composition : — 



9 RD=' Reds 

 S Rd = Whites 

 S Dr = Whites 

 I dr = White 



Further, it will be seen that there are five kinds of white and four of 

 red plants, viz. — whites of the composition : — 



Rrdd, ddrr, RRdd, rrDD, rrdD, 



and reds of the composition, 



RRDD, RrDd, RrDD, RRDd. 



Of the red it is at present only possible to distinguish three 

 kinds, viz., 



RRDD, RrDD, or RRDd and RrDd. Of these RrDd we know as 

 the parent or type, the pigmentation of which is weak. 



RrDD or RRDd has been raised twice out of " Flourball " seedlings, 

 and each case has given red and white tubered seedlings in the propor- 

 tion 3:1. Thus, 



Family A 70 red 27 white 

 „ G* 12 „ 5 „ 



The colour of the tuber RrDD is distinctly stronger than the colour 

 of the ordinary " Flourball." There is good reason to hope that the 

 type RRDD will be isolated this season : such a potato will breed true 

 to red. "Reading Russet," a pale red, selfed in 1909 and planted out 

 this year, already gives evidence of a 9:7 ratio. Amongst the whites 

 no certain distinction has yet been made between the possible kinds, 

 nor have two whites been yet successfully mated ; an experiment which 

 when the two whites contain, one the R factor and the other the D 

 respectively, will probably give rise to a coloured potato*. 



* This year, 1910, a laige number of crosses between Tarioos whites have been 

 effected* 



