SUED IMPROVEMENT MEETING. 211 



ation after the cross may be an intermediate between the two 

 parent forms and is susceptible to disease, but in the second 

 generation a large number of forms are produced in which all 

 possible combination of characters may be found. Biffin (9) 

 in his work on rust-resistant plants found the proportion of 

 resistant plants to susceptible ones to be as i to 3, indicating 

 that resistance is recessive to susceptibility. Of course, in the 

 recombination of characters, a large number of the plants pos- 

 sessing resistance will be found to have inherited sonie of the 

 undesirable characters possessed by one of the parents, but the 

 chances are good that a few plants satisfactory in all respects 

 may be found. The seed from these should be saved and all 

 undesirables discarded. The following year this seed should be 

 planted where the plants will be subjected to infection and 

 treated in all respects as outlined above for the resistant plants 

 obtained by selection alone. 



The disadvantage of this method is that if the parent plants 

 differ markedlv in their various characters so manv combina- 

 tions result in the progeny that it is difficult to obtain a satis- 

 factory plant except after several years of breeding. Orton (10) 

 calculated that the chance of securing a desirable individual in 

 the case of a cross between a watermelon and a citron was 

 as I to 4096. Yet he obtained such a desirable plant after a 

 second cross between the hybrid and the watermelon and such 

 plants are to-day able to grow upon soil infested with the wilt 

 fungus. Biffin (9) was able to obtain desirable strains of wheat 

 immune to the yellow rust by crossing immune but otherwise 

 undesirable varieties with susceptible varieties possessing other 

 characters he wished to perpetuate. 



In most of the cases of disease-resistant plants obtained either 

 by selection or by hybridization, the plants are not completely 

 immune. The disease against which they have been bred to 

 be resistant may appear on them in reduced form. They are, 

 however, practically immune and the danger resulting from 

 an attack is reduced to a minimum, in fact, as much as is usually 

 obtained from successful spraying. 



9. Biffin, R. H. Studies in the Inheritance of Disease Resistance. 

 Jour. Agr. Sci. 2 : 109-128, 1907. 



10. Orton, W. A. On the Theory and Practice of Breeding Disease- 

 Resistant Plants. Am. Breed. Assoc. Rept. 4 : 144-156, 1908. 



