E. R. Saunders 353 



different types of families in F„. We may state the expectation in 

 general terms thus : 



{a) Nearly half the F^ plants will be derived from the union 



of XYW ovules with XYW pollen ; these will give only singles in F^, 

 all with white plastids. 



(6) A small percentage of the F^ plants will be derived from the 



union of XyW ovules with XYW pollen, and these should give a 

 mixture of singles and doubles both with white plastids. 



(c) Half of the F^ plants will be derived from the union oi xYw or 

 xyw ovules with XYW pollen, and these may be expected to give all 

 four forms in F„ (i.e. singles and doubles with white plastids and singles 

 and doubles with cream plastids). 



The results obtained may be summarised thus (for details see 

 Table V). 



A total of 128 F2 families were raised, composed as follows : 



(1) 65 families were composed entirely of singles with white 

 plastids. Of these no doubt some should be disregarded on account 

 of the small number of plants recorded. Leaving out of account all 

 families of less than 10 individuals there remain 46, representing 

 a total of 1303 individuals all single and all with white plastids (see 

 paragraph (a) above). 



(2) 63 families included a mixture of singles and doubles. 



(a) Two of these contained only plants with white plastids, but in 

 both the numbers were small and included only one double, so that 

 although they may represent the type of family given above under 

 (6) the evidence is insufficient for proof. 



(/3) 11 families included all four forms, singles with white plastids 

 and doubles with cream plastids being largely in excess. 



(7) Of the remaining 50 double-containing families there were 

 22 in which singles with cream plastids and 2 in which doubles with 

 white plastids were absent (the other three forms being present in each 

 case); and 26 in which singles with white plastids and doubles with 

 cream plastids only were present. 



If we apply the same reasoning here as in the case of Mating 3, 

 where cZ-cream was used instead of (^-sulphur-white (see p. 344), we 

 shall conclude that many (? all) of these 50 families would yield the 

 missing forms if a larger sowing were made. For here as in the earlier 

 case the great majority of the double-throwing cross-breds will be de- 

 rived from the mating of xyw ovules and Z FIT pollen; recombination 



