In higher plants, the collapse of immature seed is a frequent cause of sterility following interspecific hybridization or crosses between diploid and its auto-polyploid plants.In one or both sides of interspecific reciprocal crosses : Galeopsis (Muntzing 1930(Muntzing , 1933 In the present paper, the embryological changes leading to the failure of seed development in the reciprocal crosses between B. chinensis and autotetraploid B. pekinensis are described. MATERIALS AND METHODSAutotetraploid B. pekinensis Rupr, cultivar. Chiifu-hakusai and diploid B. chinensis 1) 2)Contribution from the

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In most interspecific hybridization among diploid Brassica species (n = 8, 9, 10), few or no hybrids are obtained, and the three crosses of tetraploid (n = 17, 18, 19) by diploid species are usually only partially successful, especially in one of the reciprocal crosses. Such a cross-incompatibility complex is explained in this paper by a polar-nuclei activation (PNA) hypothesis which proposes two reversible functions, a normal reproduction and a sexual barrier, of double fertilization in higher plants. The first, i.e. the relative activating value (AV) of diploid species, was estimated as 1.0-3.5, whereas that of tetraploid species was 2.7-5.2. In addition, crosses with a polar nuclei activation index (AI) of about 15-87 % were compatible but those beyond the two limits were incompatible, 50 % of those showing normal seed development after self-pollination.