[THOMPSON] RIPENING PERIODS IN WHEAT 83 



5. Discussion and Conclusions. 



(i) Apparent Dominance in F\. 



It was pointed out in describing the experimental data that 

 lateness is apparently dominant in the Fi generation, most of the 

 plants ripening with the late parent. Nevertheless the F2 plants 

 show that we are really dealing with a case of blending, the great 

 majority of the plants being intermediate between the parents. If 

 this is true we should also have found blending, not dominance, in 

 the Fi generation. 



Yoshino (1915) obtained somewhat similar results in the case of 

 •peas but the Fi plants were not quite so late as the late parent. He 

 interprets the result as the effect of imperfect dominance, though 

 the data from later generations seem quite opposed to this inter- 

 pretation. 



Castle (1916) explains the apparent dominance of lateness in 

 Yoshino's peas as due to the vigor of crossing. The experiments of 

 East, Schull, Castle and others have shown that first generation 

 hybrids are often more vigorous than either parent and that this 

 added vigor disappears in later generations. Castle holds that this 

 added vigor of growth has prolonged the hereditary ripening period 

 of the Fi plants which is intermediate between those of the parents. 

 In the second generation, this added vigor having disappeared, the 

 plants return to the intermediate position. 



This appears to be the most logical explanation of the present 

 case, and yet there are difficulties in the explanation. It seems 

 strange that in almost every case there should be just sufficient pro- 

 longation to make Fi plants coincide with the position of the late parent 

 and even approach the mean of the late parent. One would expect 

 on the basis of the suggested explanation that among the many crosses 

 made (not all of which are given in the tables) a large proportion would 

 fall short of or go beyond the parental condition. Moreover in many 

 of the crosses there is no other indication of added vigor. The Fi 

 plants in these cases are not larger or stouter than the parents. 



In spite of these difficulties the explanation given appears to be 

 the most probable one in the present case. The disappearance of 

 true dominance after a single generation would be impossible to 

 explain. 



(2) The Multiple Determiner Hypothesis of Blending and the Constancy 

 of the Mendelian Determiner. 



It is not necessary to review the work leading to the present 

 difference of opinion touching the explanation of blending inheritance 



