HYBRID VIGOR OR HETEROSIS 



245 



productive than the original variety before it was inbred. 

 It will accordingly not be profitable to propagate these pure 

 lines as field crops, and further the amount of seed which 

 they will yield if cross-pollinated will not be large. Hence to 

 produce a large quantity of cross-bred seed will be expensive. 

 But from a small number of Fi plants a very large yield of Fo 

 seed might be obtained at small expense, since Fi plants are 

 extremely productive. The aim should be therefore to cross- 

 breed Fi plants. This can be done by securing four different 

 inbred lines and crossing these in pairs, A with B, C with D. 

 There will result two unrelated and vigorous Fi groups, AB 

 and CD which may now be planted in alternate rows. One 



AgeinDaya 40 80 120 160 20O 240 2»0 320 360 400 



Fig. 151. Growth curves of race B and Catia cutleri females and their Fi and Fa female hybrids. 



of them, if detasseled, will be naturally pollinated by the 

 other and consequently all the seed which it produces will 

 be crossbred, representing combinations of factors found in 

 AB with the allelomorphic factors in CD. Such seed, if 

 planted, will produce a field crop of maximum yield, since 

 all plants will be cross-bred Fi individuals, though produced 

 by Fi plants. This last fact will keep down the cost of pro- 

 ducing the seed because the yield will be heavy, half the 

 total crop from the area planted (see Fig. 152). 



