NATURE OF PLANTS 135 



better, we believe — showed these characters. We now see that 

 a plant may be pure bred because of its ancestors but it may be 

 just as pure bred when derived from a hybrid, because the 

 gametes are never hybrid, i. e., they always have the unit char- 

 acters segregated. The gametes derived from the first or any 

 other generation of hybrids have just as pure black and yellow 

 unit characters as those of the original parents. The application 

 of these principles of heredity have already been of the greatest 

 benefit to plant and animal breeders. Formerly years of labori- 

 ous selection of the best stock was practiced to improve a given 

 variety. It is evident now that all the possible forms which 

 can come from a cross between two fixed strains must appear 

 in the second generation. This generation suffices to produce 

 and fix a new form, and a third generation is required to separate 

 this for breeding. The desired results can be obtained in three 

 years with greater certainty than was possible formerly with 

 twenty or thirty years of breeding. Unfavorable characters may 

 be removed or desirable ones added to a stock with such certainty 

 that horticulturists have advertised new forms, describing their 

 characters and qualities before they have been bred. Late fruit- 

 ing grains have been made to mature earlier, species susceptible 

 to cold have become hardy, as in the case of wheat which may 

 now be cultivated in Canada many miles north of its former range, 

 and non-resistant forms have become immune to disease. The 

 annual loss from a single plant disease, one of the rusts, is esti- 

 mated at 500 million dollars. In the same way the percentage 

 composition of the reserve food in grains has been changed and 

 the yield increased with the result that the productivity of the 

 soil will be increased by untold millions. 



How applicable Mendel's law is to all cases of crossing has 

 not been determined. In the case cited above where only two 

 unit characters were involved there can be but three combi- 

 nations of them, i. e., pure black, pure yellow and a mixture of 

 black and yellow. It is manifest where many unit characters 

 appear that there will result a re-combination of these characters 

 in the offspring that is complicated and difficult to interpret. 

 When however the number of unit characters that appear in any 



