128 



Heredity and Eugenics 



strawberries, potatoes, pineapples, and many other kinds 

 of economic plants belong to this category. This is a great 

 advantage. One can so propagate homozygous strains if he 

 wishes, but in addition he has a means of utilizing heterozy- 

 gotes that would not breed true to seed and also of keeping 



the greater vigor that ac- 

 companies heterozygosis. 



No better example of 

 such work can be given 

 than that of Webber on 

 citrus fruits. The great 

 bugbear of the Florida 

 orange grower is the frost 

 that occasionally comes, 

 leaving devastation in its 

 wake. Webber, therefore, 

 set himself the definite 

 problem of producing a 

 frost-resisting orange. He 

 made several reciprocal 

 crosses between the com- 

 mon orange and the hardy 

 but worthless trifoliate 

 orange {Citrus trifoliata) . 

 Among the seedlings ob- 

 tained, several have proven 

 valuable. They form a new class of citrus fruits and have 

 been called Citranges. Three of these varieties have been 

 named the Rusk, the Willits, and the Morton. The Rusk, 

 which is a hybrid of orange crossed by trifoliata, is a small 

 fruit with a bitter tang like the pomelo. It makes excellent 

 marmalade and preserves. The Willits, coming from a 



Fig. 55. — Intermediate character of 

 an Fi hybrid. At left, Nicotiana panicu- 

 lata; in center, hybrid; at right, Nicotiana 

 alata. 



