428 Report of the Department of Horticulture of the 



this stimulation due to hybridity may be analogous to that of 

 ionization." 



Shull further states : "A. B. Bruce proposes a slightly different 

 hypothesis in which the degree of vigor is assumed to depend upon 

 the number of dominant elements present rather than the number 

 of heterozygous elements." Bruce's view harmonizes with the one 

 given by Keeble and Pellew. 



In addition to the benefit already noted as obtained from cross- 

 ing, there are obtained others of lesser importance but probably 

 correlated with the increased vigor, as, for example, early 

 flowering, early maturity, hardiness and lessened liability to 

 premature death. Darwin (10) cites many instances in which 

 the crosses have flowered earlier than plants from self-fertilized 

 seed — and a few where the reverse order has taken place. 

 Cyclamen 'persicmn is a marked case of premature flowering, for 

 during two successive seasons a crossed plant flowered some weeks 

 before any of those from self -fertilized seed in all four pots. 



The early maturity of fruit borne by crossed tomato plants is 

 discussed in the text of this bulletin. 



The increase of hardiness of crossed plants was found by Dar- 

 win to be very marked in ISTicotiana and Ipomea, both of which 

 resisted the cold and inclement weather much better than the self- 

 fertilized plants. " The offspring of plants of the eighth self- 

 fertilized generation of Mimulus crossed by a fresh stock, survived 

 a frost which killed every single self-fertilized and inter-crossed 

 plant of the same old stock." EsclischoUzia, already noted as an 

 exception, was hardier Avhen not cross-fertilized. Self-fertilized 

 seedlings of Beta vulgaris were found to perish beneath the ground 

 in large numbers, when the crossed seeds sown at the same time 

 did not suffer. These observations of Darwin, in addition to 

 others on the behavior of self-fertilized seeds of the petunia give 

 ample proof that hardiness is affected by crossing. 



Since cross-fertilized plants have a greater resistance to ex- 

 tremes in climatic conditions, and as they are generally more 

 vigorous than their self-fertilized brethren, it is not unreasonable 



