UTILIZATION OF HYBRIDS IN PLANT BREEDING 365 
easily distinguished by their larger size while still quite small, all that 
the nurseryman has to do is to select the hybrids for budding or grafting. 
Sterility or partial sterility is frequently associated with increased 
vigor in first-generation hybrids between species. The large flowers 
and luxuriant growth of some of the sterile tobacco hybrids render them 
promising subjects for use as ornamentals. Partial sterility, when mani- 
fested by a lessened production of seed, may not be accompanied by 
any decrease in yield of fruits. In such cases therefore it is a positive 
advantage if the plant can be propagated vegetatively. 
Rapid-growing timber and ornamental trees of a number of different 
species have been produced by crossing distinct forms. Henry men- 
tions the following valuable trees which, on account of their vigor, 
botanical characters and non-occurrence in the wild state, are presumably 
first-generation hybrids: black Italian poplar, London plane, Huntingdon 
elm, cricket-bat willow and the common lime (Tilia vulgaris). Accord- 
ing to Henry the pioneer work on hybridization of trees was done by 
Klotzsch at Berlin in 1845. He crossed two species each of pine, oak, 
elm and alder. He “claimed that by hybridization, both the rapidity 
of growth and the durability of timber of forest trees could be augmented 
considerably; but no further experiments were made, and his pioneer work 
fell into oblivion.”’ The art of breeding trees was renewed by Burbank’s 
work with the walnuts about 1890. Henry reports results with F'; hybrids 
in Populus, Fraxinus, Alnus, Ulmus and Larix. He points out that one 
of his most vigorous hybrids (Populus generosa) was ‘derived from two 
parents so little related that they are placed in two distinct sections of the 
genus.”” At the same time, ‘‘a cross between two races of the common 
alder shows considerable vigor, though the parents are so closely allied 
that they can only be distinguished by the most trivial characters.” 
Thus it appears that prediction as to the outcome of species crosses in 
trees is quite as impossible as in other classes of plants. There is great 
need for further experimentation. In planting wind-pollinated species 
provision can easily be made for natural hybridization by mixing groups 
of different species. It has been found that the quality of the timber in 
rapid growing F; hybrids is equal or superior to that of the parents. 
Increased resistance of fF; hybrid plants to insect pests and diseases 
is doubtless often merely another manifestation of their increased vigor. 
But in this connection it is to be remembered that disease resistance is 
generally a heritable character, so that in a particular instance its ap- 
pearance in F’, will depend on the factorial composition of the parents and 
the relation of the factors in inheritance. 
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