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 F\ 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 FI will depend on the factorial composition of the parents and 

 the relation of the factors in inheritance. 



