410 



GENETICS IN RELATION TO AGRICULTURE 



typic relation of aphis resistance and susceptibility in these plants, but 

 the indications are that these characters are conditioned by a single 

 factor difference. Another interesting case of the inheritance of resist- 

 ance to aphis was observed by Gernert in FI hybrids between teosinte and 

 corn. Both the corn root-aphis, Aphis maidiradids, and the corn plant- 

 aphis, A. maidis, were involved, and both the teosinte and the hybrids 

 were completely resistant while the corn was badly infested. The desira- 

 bility of securing aphis resistant varieties of maize will be apparent when 

 it is realized that most of the corn growing regions of North America are 

 infested with these insects and that the loss in reduction of yield caused 

 by them is enormous. 



The work of Van Fleet on hybridizing various species of chestnuts 

 was begun 10 years before the terrible bark disease had worked havoc 



FIG. 167. In the center is a nut produced by a cross between the American bush 

 chinquapin, Costarica pumila, (right), and the Japanese chestnut, C. crenata, (left). Al- 

 though intermediate in size the hybrid nut is disease resistant and of good quality. (From 

 The Journal of Heredity.) 



C 



with the chestnut trees near New York City, which is the oldest known 

 center of infection. Hence many crosses were made with either the 

 American or European chestnut as one parent, but in 1907 these were all 

 destroyed by the Endothia. Fortunately however numerous controlled 

 pollinations were made on the bush or Virginia chinquapin, Costarica 

 pumila, using pollen of a Japanese species, C. crenata,' as well as other 

 Asiatic chestnuts. It is asserted that the Asiatic species and the chin- 

 quapin-Asiatic hybrids are highly resistant, because few have shown any 

 appearance of infection although surrounded by diseased trees, and that 

 even when infection takes place the injury is quite local in character. 

 Van Fleet adds that second generation seedlings of chinquapin-crenafa 

 crosses show no disease although constantly exposed to infection (Fig. 

 167). Thus a beginning has been made in what promises to be an import- 

 ant branch of nut breeding, and the orchard production of commercial 

 chestnuts has been insured against future encroachments by a deadly 

 disease through the timely efforts of a zealous and far-sighted plant 

 breeder. 



