E. R. Saunders 



305 



;io-(2-siiigle 



pollen X ovules 



d-single 



Fi singles 



I 



jio-d-single 



pollen X ovules 



singles 



I 



F2 singles singles 



I 1 r 



singles doubles sing! 



i I 



r"~:T 1 



sing 



I 



es doubles 



F3 singles singles singles singles doubles singles singles singles doubles 



As there is no reason to suppose that the ovules and pollen of the 

 no-d-singles are unlike in constitution, this difference in behaviour of 

 the ^1 crossbreds resulting from reciprocal unions must be due to 

 a difference in the composition of the ovules and pollen produced by 

 the d-singles; and the experimental data are in accordance with the 

 explanation already given, viz. that the ci-singles produce two kinds of 

 ovules, but only one kind of pollen grain'. 



Moreover this interpretation is confirmed by the fact that doubles 

 are always produced in F^ from a cross between two c?-singles, whereas 

 doubles are never obtained in F-^ wheu the mating is between a d- and 

 a no-d-single. Doubleness in this respect behaves as a recessive. 



So far the case is clear, and the explanation just given has been 

 amply borne out by subsequent experiments. But certain points in the 

 relations existing between singles and doubles still remained obscure. 

 Though it was now clearly established that the appearance of doubles 

 in Stocks is exhibited in an orderly and definite manner, and is entirely 

 independent of external conditions, it still remained to determine the 

 proportion of doubles thrown by the eversporting singles, and to 

 ascertain, if possible, whether this proportion is constant. Doubleness 

 behaves as a recessive to singleness ; how then are we to account for 

 the production of doubles in excess ? Nor is doubleness the only 

 character which behaves in this remarkable way. In a certain race of 

 double-throwing singles, viz. sulphur-white, the plants are also ever- 

 sporting in regard to plastid colour; every individual yields both whites 



' The conception of a difference in constitution between the ovules and pollen grains 

 of a plant was first put forward in 1908 in connection with the Stocks. It is interesting 

 to find that a difference in reciprocal crosses among certain forms of Oenothera has led 

 de Vries to the conclusion that differences between the ovule and pollen series of the 

 same plant may also occur in this genus. (Cf. de Vries, Biol. Centr. 1911.) 



