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 F, plants it is possible to produce even higher yields in F; 
and Fy. 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 F; 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- 
azygous strains which can then be utilized in the production of the most 
vigorous 7’, 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 /'; 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. californica); while the Paradox type comes from crossing the 
walnut of commerce (J. 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 
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