CENOTHERA AND THE MUTATION THEORY 311 



supposed to account for the types named, such 

 differences do not offer an explanation of most of 

 the other new types nor of the pecuhar behavior 

 shown by species crosses in the genus (Enothera. 

 As will appear below these two classes of phenomena 

 both suggest strongly that the plants are heter- 

 ozygous, in spite of the fact that they breed nearly 

 true. 



If O. biennis and O. syrticola (muricata) are 

 crossed, the Fi hybrid strongly resembles the male 

 parent, whichever way the cross is made. Com- 

 binations of these hybrids with each other and with 

 both parent species have shown that the pollen 

 and egg cells of each species carry different genes. 

 The chief characteristics in which the two species 

 differ are transmJtted only through the pollen, and 

 only the minor differences between the hybrids and 

 their respective fathers are due to genes transmitted 

 by the eggs.^ The hybrids themselves behave in 

 the same way so far as these characteristics are 

 concerned; what they received from their father 

 they transmit only through their pollen, what they 

 received from their mother they transmit only 

 through their eggs. As was pointed out by deVries, 

 this phenomenon is perhaps to be connected witli 

 the fact, discovered in 1901 by Geerts, that about 

 half the pollen grains and alx)ut half the ovules 

 degenerate in certain of the Oenotheras — apparently 



1 These two species are quite similar, so that the characters transmitted 

 in this peculiar fashion constitute only a small proportion of m11 the 

 characters of the plants. 



