156 INBREEDING AND OUTBREEDING 



are not attacked by the aphids which damage maize. The 

 cross between the inbred strain of maize most susceptible 

 to smut, previously mentioned, and the strain not affected, 

 gives a hybrid which is only slightly parasitized. The 

 same thing has been noted in crosses between other 

 strains of maize, some of which are quite badly damaged 

 by an unidentified leaf blight organism. Radish seedlings . 

 which were naturally cross-fertilized were much less dam- 

 aged by damping-off fungus than uncrossed seedlings 

 from the same plants. Resistance is not shown by all 

 first generation hybrids when the parents differ in suscep- 

 tibility. Some cases are known in which the hybrids are 

 fully as susceptible as the less immune parent. In 

 the majority of crosses reported, however, in which re- 

 sistance to parasitism is a factor the hybrids tend to 

 show resistance. 



Among the diverse manifestations accompanying 

 heterozygosity may be mentioned viability of seed. In 

 maize, crossed and selfed seeds from the same ears have 

 shown a difference of 16 per cent, germination in favor 

 of the crossed seeds. The crossed seedlings appeared 

 earlier and grew faster from the first. 



Increased facility of vegetative propagation of hy- 

 brids was frequently noted by the early hybridizers. 

 Sage-ret 191 makes particular note of a hybrid tobacco 

 which easily propagated itself vegetatively. Many of our 

 cultivated fruits which are propagated by buds, grafts, 

 cuttings, etc., owe part of their excellence at least to the 

 fact that they are in a heterozygous condition. Moreover, 

 there is no evidence to prove that plants lose any of their 

 hybrid vigor in long continued vegetative multiplication 

 through innumerable generations. 



