114 Heredity and Eugenics 



is an exception, however— a thing apart from the usual 

 breeding procedure. 



It will be remembered that when a plant receives iden- 

 tical character factors from each parent, that character is 

 homozygous and breeds true; but when the plant receives 

 the character from only one parent, the character is hetero- 

 zygous and shows segregation in the next generation. This 

 heterozygous condition, though not fixable itself, since it 

 always breaks up in the succeeding generation, is a valuable 

 asset to the plant breeder if properly utilized and a distinct 

 disadvantage if unrecognized. 



The fusion of two gametes into a zygote which is known 

 as fertilization effects two very different results: first, a 

 union of the hereditary factors possessed by these gametes; 

 second, a stimulation to the cell division necessary for 

 normal development. Probably in every case where fer- 

 tilization can take place at all there is a certain amount of 

 this stimulus to development, but the fact of especial interest 

 to plant breeders is that this stimulus is generally far greater 

 in a hybrid or heterozygote than it is in a pure-bred or 

 homozygous individual. The stimulus is simply toward 

 greater and quicker cell division and affects only size and 

 rapidity of maturity. 



The tobacco genus {Nicotiana) furnishes an admirable 

 type illustration of the stimulus due to heterozygosis because 

 the species are generally self-fertilized under natural con- 

 ditions. The stamens and the pistil are about the same 

 length, and since the pollen is usually shed before the flower 

 opens, self-pollination must occur unless foreign pollen is 

 carried to the unopened bud by insects. When varieties 

 of the common tobacco N. tabacum are crossed together, 

 the first hybrid generation is nearly always from 5 to 50 



