356 Douhleness in Stocks 



VIII. Summary. 



In the preceding pages an attempt has been made to work out 

 a scheme which will account for the behaviour, so far as we know it 

 at present, of various races of Stocks in regard to the two characters, 

 colour of plastids and production of doubles. The results have already 

 shown that the relationship of the various factors concerned is by no 

 means simple, but it is not unlikely that as more facts come to light 

 still further complications will become apparent. It may be claimed 

 however that the scheme as it stands affords a useful working hypothesis 

 enabling us to grasp a complicated series of facts ; moreover it is one 

 which can be tested in detail by further experiment on definite lines. 

 A solution which enables us to fit together so many pieces of the 

 puzzle must, one cannot but believe, prove to be substantially correct. 



We may therefore venture to add to the conclusions already 

 formulated on pp. 321 — 324 the following general statements : 



(1) All sap-coloured races of Ten Week Stocks so far investigated 

 (i.e. azure, light purple, dark purple, marine blue, flesh, copper, red) and 

 the two non-sap-coloured forms pure white and cream can occur under 

 two forms, a pure-breeding form and an eversporting form. 



(2) The sulphur-white race — a race which is peculiar in being 

 eversporting in regard to plastid colour as well as in regard to douhle- 

 ness- — is only known in the double-throwing form. It produces single 

 whites, double creams and a small percentage of double whites. 



(3) Eveiy individual in an eversporting strain yields doubles in 

 excess ; the proportion may be stated as 7 + x single to 9 — x double 

 where *■ is less than 1. 



(4) All the pollen grains of such strains appear to carry douhleness: 

 that is to say, in these strains the distribution of the factors for single- 

 ness {X and Y) is limited to the gametes of one sex. 



(5) The ovules in every individual belonging to these strains are 

 mixed, the proportion of those carrying singleness and douhleness is 

 presumably the same as the proportion of singles and doubles among 

 the offspring, since the pollen is uniform. 



(6) The proportion of 7 + x single to 9 — x double is most easily 

 explained on the supposition (1) that two factors at least are con- 

 cerned (X and V), (2) that the zygote is heterozygous in regard to 

 both, and (3) that in the case of the ovules these factors show partial 

 coupling of the kind with which we are already familiar in the Sweet 



