The author has tried to accumulate data on the reproduction modes of crop plants: autogamy and allogamy in the case of sexuality, involving self-fertility and self-sterility, and different means of vegetative propagation and apomixis. In combination with the state of ploidy and the basic chromosome number the different modes of reproduction exert a considerable influence on population structure and the success or failure of different methods applied in plant breeding. This relates to the use of selection, hybrid vigour (F1 heterosis), gene recombination, as well as polyploidy and induced or spontaneous mutation. It is pointed out that extranuclear (cytoplasmic) inheritance should not be neglected as a device also in the case of polyploidy and mutation.Transitional stages exist between autogamy and allogamy. Autogamy is obligate in no or at least very few cases. In allogamous species inbreeding and subsequent outcrossing are important features in their improvement by breeding. In dioecious, monoecious and hermaphroditic species the modes of reproduction can be switched into one another by appropriate methods of gene recombination, mutation and selection. Apomictic species, for instance several grasses, display a series of transitions between more or less obligatory apomixis (parthenogenesis and vivipary) and partial or complete sexuality.At the end of the article data are presented to indicate how various modes of reproduction influence the methods applied in the exploration and conservation of plant gene pools.Finally, the pioneer work on plant exploration carried out byVAVILOV, ZHUKOVSKY and their co-workers is emphasized. Favourable genes, chromosomes and cytoplasms present in natural populations have to be preserved. New favourable genes etc. should be continually produced by mutation. Preservation of old genes and induction of new genes are means of augmenting the breeders' resources in their efforts of continuous crop plant improvement.