B. R Saunders 327 



that in case 2 the result will be the same as in case 3 which is already 

 known ; while in raatings 5 and 6 the result will presumably be the 

 same as if the seed parent had in each case been self- bred instead of 

 cross-bred. We have then to consider the results in matings 1, 3 and 

 4 in the light of the assumptions made above. 



Mating 1. rf-sulphur-white ? x d-non-cream ^. 



Five diflferent matings of this kind were made (see p. 326). 



This type of mating gives only white plastids in JP,, a result fully in 

 accord with expectation. For the sulphur-white ovules which carry 

 creamness, by assumption, also carry doubleness ; hence when this race 

 is crossed with any non-cream form w^hose pollen carries doubleness, 

 cream will not presumably reappear in any succeeding generation. It 

 will have been bred out completely, though in the recessive condition, 

 in the F^^ non-cream doubles. Thus only those sulphur-white ovules 

 which carry the white plastid factor give rise to singles in F^. These 

 F^ singles are therefore all homozygous as regards the factor TF, since 

 the mating will have been between XYW ovules and xyW pollen; 

 segregation in their case may therefore be expected to proceed on the 

 same lines as in a pure-bred d-race with uncoloured plastids, and to 

 yield a similar proportion of singles and doubles. As regards the 

 question whether a ratio of 7*5 s. : 8"5 d. or 7 s. : 9 d. more nearly repre- 

 sents the facts, it happens that the numbers obtained in this case agree 

 better with the latter alternative (7 s. : 9 d.). It is just worth noting 

 however that the mating in which the largest record was obtained, both 

 absolutely and in proportion to the quantity of seed sown, and in which 

 therefore the result might be supposed to be the most reliable, viz. the 

 mating with d-glabrous red, leaves either alternative equally probable. 

 In three out of the five matings the experiment was carried to ^3, 

 where the complete breeding out of the cream was further confirmed, 

 but where again the evidence is not quite decisive as between the two 

 ratios. For, though the sum of the three totals gives 7 s. : 9 d., one of 

 the two larger sowings gives almost exactly 7'5 s. : 8'5 d. The results 

 are summarised below (see p. 328). 



We may then conclude that in the unions of the form rf-glabrous 

 sulphur-white % x d-glabrous non-cream <^ segregation in single Fi 

 plants and in later self-bred generations is like that in any pure-bred 

 eversporting glabrous form with white plastids. 



28—8 



