W. Nkilson Jonks 87 



Also when (BxM) is pollinated by (MxB), the factors involved 

 will he B ^ from the {B x M) ovules and B ^ from the {M x B) pollen 

 grains. The resulting zygote having the constitution JB ^ jB ^^ re- 

 sembles pure biennis. Thus the hypothesis suggested by de Vries fita 

 the facts observed in (Enothera completely. The only criticism to be 

 brought against it is, that although sterility of half the ovules and 

 pollen grains is known to occur in many (Enot/iera species", there is at 

 present no proof that the sterility is selective. 



The behaviour of other species of (Enothera when hybridised would 

 appear to be explicable in the same way. 



A similar explanation, depending on a difference in constitution 

 between ovules and pollen graius, might be advanced to account for the 

 behaviour of the Digitalis hybrids; only in this case it would be the 

 ovules which carried characters most like the parent plant instead of 

 the pollen grains as in (Enotliera. What the result of crossing the 

 Digitalis hybrids together might be, and whether they would behave 

 similarly to the (Enothera hybrids, it is impossible to say, since the 

 crosses proved infertile with me. 



Seed however was obtained as the result of pollinating the hybrids 

 by their parents, and the resulting seedlings may throw some light 

 on this problem, although they are as yet too young to show any 

 characteristic features. 



The inheritance of doubleness, &c., in Stocks has also been explained 

 on the basis of a difference between pollen gi-ains and ovules^. 



The phenomena exhibited by (Enothera and Digitalis are not 

 necessarily, of course, of the same kind : and it is possible that the 

 resemblance of the reciprocal crosses to the pollen parent as seen in 

 the former is due to a difference in constitution of pollen grains and 

 ovules, while their resemblance to the seed-parents in the latter is due 

 to the influence of the cytoplasm of the egg-cell (either direct or 

 indirect). 



I wish to express my thanks to Dr Keeble for his valuable help and 

 criticism and to my colleague MLss M. C. Rayner, who is also responsible 

 for Plate IV. 



^ J. M. Geerts, "Beitrage zar Kenntniss der Cytologic and der partiellen Sterilitat von 

 (E. Lamarckiana," Recueil Trav. Bot. Neerlandais, v. p. 93, 1909. 

 * E. R. Saunders, Journal of Genetics, i. 4, 1911. 



Joam. of Cren. u 



