GENETICS IN ITS APPLICATION TO PLANT BREEDING

SUMMARY 1 The importance of Mendelism in relation to practical breeding problems is illustrated by reference to early work on the inheritance of economic characters. 2 Examples are given of improved varieties that have been produced by individual plant selection. The possibilities of success are shown to be dictated by the completeness of the material and one of the essentials for successful breeding is shown to be the collection of adequate and representative stocks of initial breeding material. This is illustrated largely by reference to Russian work, which has also elucidated the laws governing the distribution of plant varieties, characters and genes. 3 The possibilities of putting interspecific or distant crosses to practical use are analysed and cases where this has been done are cited. Chromosome duplication is shown to be a frequent phenomenon in such cases and to be a factor greatly increasing the economic value of such distant crosses. Numerous cases are quoted and the phenomenon is shown to have played an important role in the origin of new species and forms in nature. Its experimental control is thus of great practical value. 4 The origin of new forms of pathogenic fungi by hybridization and mutation is illustrated and shown to be a serious obstacle in breeding for disease resistance. 5 Examples are given where the obstacle has been overcome by breeding methods and other cases are mentioned where the problem of disease resistance remains unsolved. 6 The role of mutations in the origin of new forms of agricultural plants is discussed and the possibilities of effecting this process artificially are analysed. 7 The phenomenon of vernalization is described, with special reference to breeding. The significance of this and other recent physiological studies in producing forms possessed of earlier maturity is discussed. 8 The nature of plant yield is discussed and reference is made to attempts to analyse and regulate this by genetical methods.