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 F'; hybrids between teosinte and 
corn. Both the corn root-aphis, Aphis mazdiradicis, and the corn plant- 
aphis, A. mazdis, 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 
Fic. 167.—In the center is a nut produced by a cross between the American bush 
chinquapin, Castanea 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.) 
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, Castanea 
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-crenata 
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. 
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