1907] GATES—HYBRIDIZATION OF OENOTHERA MUTANTS 



5 



A 



B 



Heterozygote 



A 



Constant 



Extracted dominant 



A(B) 



(50%) 

 Splitting 



Heterozygote 



B 



(25%) 

 Constant 



Extracted recessive 



Fig. 2. — Mendelian hybrid 



Two significant exceptions to this form of hybrid among the 

 mutants are found in O. hrevistylis and O. laevifolia, which never 

 appeared as mutants in DeVries' cuhures, although he believes them 

 to have formerly mutated from O. LamarcUana. They form with 

 O. Lamarckiana ordinary MendeHan hybrids. 



J- he fact that the percentage of a mutant appearing in a cross can 

 be experimentally varied in different ways is also of importance in 

 this connection. Thus the percentage of the parent forms appear- 

 ing m a cross varies with the position on the stem of the particular 

 flowers crossed. The plants bloom from below upward on the stem, 

 and DcVries observed that after the first few weeks of blooming 

 there is a steady and marked decrease in the size of the flowers bloom- 

 ing, until at the close of the season they are but little larger than those 

 01 O. biennis. I have observed the same thing in my own cultures 

 from seeds of DeVries at Woods Hole, Mass. At the end of Sep- 

 tember many of the plants wTre still blooming vigorously at the top, 

 and the flowers were little if any larger than those of O. biennis. 

 When pollen from a mutant is used to cross with O. Lamarckiana, if 

 It IS obtained from the earlier flowers of the season, that is from the 

 larger and more vigorous flowers which are lower on the stem, the 

 heredity-coefhcient,^ or percentage of the mutant appearing in a cross, 

 Will be larger than if later and smaller flowers are used. Similarly 

 the heredity-coefficient (Erbzahl) of flowers on the side branches of 

 • ^^^^ when crossed with O. Lamarckiana is said to be less than that 



3 I would suggest this term as an English equivalent for the German Erbzahl and 

 snail use it in this sense in the rest of the paper. 



