364 GENETICS IN RELATION TO AGRICULTURE 



results in 1909 showed the average of the hybrids to be about 185 pounds 

 more per acre than that of their parents. The maximum increase 

 obtained was 492 pounds per acre. By selecting seed from the highest- 

 yielding Ft plants it is possible to produce even higher yields in Fz 

 and F 4 . But such high-yielding selections are not fixed and under con- 

 ditions of commercial culture the yield and uniformity would undoubt- 

 edly decrease rapidly. It appears that the growing of FI hybrids offers 

 the one chance of commercial production of the highest possible yields 

 combined with uniformity in size and shape of leaf. The matter of 

 quality of cured leaf is more difficult of solution since this is a complex 

 character and is easily affected by environmental conditions. Until 

 further investigations have been made it seems that the only safe pro- 

 cedure is to choose as parents only varieties or strains that produce leaf 

 of high quality. 



Application in Vegetatively Propagated Plants. In this class of plants 

 the stimulus due to heterozygosis has been extensively utilized, but this 

 has been the result of the method of propagation rather than the conscious 

 use of the principle. In potatoes and strawberries, for example, the 

 commercial varieties are all hybrids. The crosses having been made, 

 the best plant of the first generation became the source of a new variety. 

 There are many opportunities for further application of this principle in 

 the bush and tree fruits, not only for vigor but for excellence of quality 

 as well; also in asparagus, rhubarb, hops, pineapples, sugar cane, sweet 

 potato and many ornamental plants. 



It is thought by some horticulturists that the greatest possible im- 

 provement in fruits can only be secured by preparing for hybridization 

 by several generations of inbreeding. Thus Jones, proceeding on the 

 assumption that increase of vigor in hybrids is due to heterozygosis, 

 recommends the general adoption of inbreeding in order to secure homo- 

 zygous strains which can then be utilized in the production of the most 

 vigorous FI hybrids. But it is to be remembered that only a portion of 

 the homozygous strains could be expected to produce superior F\ hy- 

 brids. It is, therefore, a serious question whether this method would be 

 as economical in the long run as the crossing of existing varieties. 



The use of F^ hybrids as rootstocks for vines, tree fruits and nuts is of 

 recognized importance. The Royal and Paradox Walnuts, which were 

 named by Burbank from specimens which he produced, furnish a striking 

 illustration. The Royal type of hybrid is produced by crossing the 

 Black Walnut of eastern states (Juglans nigra) with the California Black 

 Walnut (J. calif ornica) ; while the Paradox type comes from crossing the 

 walnut of commerce (/. regia) with either of the above named black 

 walnuts. Hybrid seedlings commonly appear in the seed beds planted 

 with seed from trees standing near trees of other species. As they are 



