414 GENETICS IN RELATION TO AGRICULTURE 
ble plants; neither are there observable differences in time of germination, 
rate of development or period of maturity. Furthermore, the resistance 
is specific; varieties that resist the wilt may be susceptible to bacterial 
blight and vice versa. : 
That wilt resistance is a heritable character was strikingly dem- 
onstrated by Orton’s creation of a wilt-resistant edible watermelon, 
Crtrullus vulgaris. All watermelons appear to be very susceptible to 
° 
Fic. 168.—Parents and product of third generation offspring of the watermelon-citron 
hybrid. (After Orton.) 
the disease. Extended tests in 1900 and 1901 failed to show any basis 
for selection among the 120 or more varieties tested. Recourse was had, 
however, to an inedible form of Citrullus vulgaris known as citron or stock 
melon which was immune to wilt. From a cross between this citron and 
the Eden variety of watermelon F; hybrids of “wonderful vigor and 
productiveness”’ were raised. The fruits were intermediate in character, 
having the oval form and stripes of the watermelon and the hard flesh of 
the citron. The F, population was extremely variable in every respect, 
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