312 



Douhleness in Stocks 



(a) Sap-coloured races. 

 i. Evidence from self-fertilisation. 

 The number of individuals tested in each case is shown below 



Number of Individuals Tested. 



Copper 

 glabrous 



Totals 

 12 



52 



13 



31 



Totals 



16 



54 



22 



108 



Twelve individuals belonging to different sap-coloured forms were 

 taken at random, and they and 96 of their descendants were self- 

 fertilised. Doubles were obtained from each of these 108 plants. (For 

 details see Table III.) Thus the evidence, so far as experiment has yet 

 gone, indicates that the double-throwing strains of these forms now on 

 the market are similar to the red and sulphur-white races in that they 

 are genuinel}' eversporting, and that it is in fact impossible to breed 

 out the doubles. 



ii. Evidence from cross-breeding. 



To obtain the further proof that the double character is being 

 carried by all the pollen in each of these sap-coloured strains necessi- 

 tates the raising of a large number of F^ plants which have been bred 

 by self- fertilisation from the mating no-d $ x c? </ where the ^ parent 

 belongs to the sap-coloured form which is to be tested. 



Up to the present 36 ^1 plants representing as many pollen grains 

 contributed by 6 c?-parents have been tested in this way. The 

 parentage of these F^ plants and the number of pollen grains tested 

 in the case of each parent are shown below ; the composition of the F^ 

 families will be discussed later (see p. 336 and Table IV); those marked 

 thus * have already been recorded (see Report II, p. 37). 



