BEGINNINGS OF THE HETEROSIS CONCEPT 



47 



which, along with many types of asexual reproduction including partheno- 

 genesis, specifically enable the organisms possessing these special mecha- 

 nisms to maintain the full advantages of heterosis. On one occasion, one of 

 my new hybrid combinations in Oenothera happened to be planted through 

 an area in my experimental field where the soil had become so impoverished 

 that none of my other cultures reached their normal growth. Many of the 



Fig. 2.10 — The Fi hybrids between a cultivated form of Helianthus anniius and a wild form 

 of the same species received from Kansas. This photograph, taken at the Station for Ex])eri- 

 mental Evolution in 1906, shows the author alfixing a glassine hag to a head of one of the 

 hybrid plants. The two parents of this hybrid averaged from 5 to 6 feet tall, while 51 of 

 these Fi hybrids, measured on August 28, 1906, ranged in height from 6.7 to 14.25 feet, the 

 average being 10.46 feet. This may be considered my iirst experience with hybrid vigor. 



