212 PROBLEMS OF FERTILIZATION 



Darwin has reported more than thirty cases of self- 

 incompatibility, sometimes absolute, sometimes only 

 relative; since then numerous additional cases have 

 been cited. The genera most carefully studied have 

 been Corydalis, Secale, Lilium, Cardamine, Antirrhinum, 

 Reseda, Nicotiana, and Cichorium. Self-sterility is prob- 

 ably never absolute in any species; it may appear in 

 normally self -fertile species sporadically (Stout), or a 

 self-sterile individual may become self-fertile under ad- 

 verse conditions (East and Park, 1917). Pollination of 

 different flowers of the same plant (geitonogamy) may 

 be slightly more successful than strict self-fertilization 

 (autogamy), but not always; in Lilium bulbiferum all 

 the plants of the same clone have been found to 

 be cross-sterile, but seed-sisters, on the other hand, 

 cross-fertile. The phenomenon may thus concern self- 

 sterility of parts of the same flower, sterility between 

 different flowers of the same plant, and sterility between 

 asexually produced offspring of the same plant. Recent 

 experiments (Correns, Stout, East) have also shown 

 that it may be transmitted like Mendelian characters 

 and thus affect in the form of cross-sterility entire 

 sections of a population. 



The study of this subject entered a new phase with 

 Correns' experiments on the inheritance of self -infertility. 

 He took two unrelated plants B and G of Cardamine 

 pratensis, which were self-sterile, and made the recip- 

 rocal crosses B? X G<3 and G? X B<3, constituting series 

 i and 2 respectively, and then investigated (i) the 

 relations of the parents and offspring to two unrelated 

 plants, (2) the relations of the pollen of both parents 

 to thirty offspring of each series, and (3) the relations 



