The following three points demand attention in transferring genes for blast resistance from indica rice varieties by the backcross breeding technique. l) Blast resistance may genetically associate with some undesirable characters. 2) The identical gene may be extensively dis-tributed in different indica varieties. 3) A special gene for resistance may be selecti-vely incorporated into backcross progenies even when donor parents carry two or more different genes for resistance. Plant materials used in the present study were lines or populations derived from japonica×indica hybrids obtained by repeated backcrosses to Japanese varieties. The breeding procedure is briefly shown in Fig.1. Indica rice varieties used as donor parents in the present backcross programs were not only resistant to blast but highly photosensitive and extremely late in flowering under the rice growing condition in Japan. Then the relation between the blast resistance and the days to heading was investiga-ted with seven first backcross populations derived from different crosses. As shown in Table 3, genetic associations were detected in the first three crosses, but no association was observed in the other four crosses. It is very beneficial to detect such an undesirable linkage in the earlier stage of backcrossing. If several crosses are available, we may be able to choose the one which is free from the undesirable linkage. When we have to adopt one of the crosses with the undesirable linkage, then we may well expand the scale of breeding or select lines segregating for blast resistance in selfing generations so as to increase tne chance to obtain desirable recombinants.

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A highly complicated peroxidative isozyme system was exhibited in the male and female flowers, buds and peduncles in a cultivar., Silver King, of Begonia rex. The peroxidase bands were classified into three groups, i.e., sexual organ-specific, age-specific, and non-specific in floral organ. The bands designated, as 2B, 3B, 3C, 2E, 4E and 3F were found to be specific to male flowers, and 2A, 1D, 3E and 4F were specific to female flowers. The bands occurring in open male flowers were 3C, 2E and 4E, whereas those specific to younger male flowers (buds) were 2C, 1D, 2D, 4D, 2F and 7F. Two bands designated, as 1A and 1B were regarded as non-specific since they appeared in all the floral organs.

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This study was started in 1967 to clarify the inheritance of several characters which were consi-dered important to breed new peach cultivar for rootstock use. This paper deals with studies on inheritance of characters of red color of leaves and dwarfness in height. The peach crosses used in this study were grown for two generation. Akame, one of the parent cultivars, is a red leaf cultivar of normal height and the other one Juseito is a green leaf cultivar of dwarfish type.

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Summer-flowering pot-plants of Achimenes are becoming recently popular in our country. In order to clarify their floral and genetical characteristics, the author has grown 6 cultivars since 1971 and then isolated a spontaneously mutated strain from a cultivar "Miyoshino", which was very tall, Iarger -leaved and giant-flowered than the original one. Four cultivars, i. e. Purple King, White Wonder, "Shisei" and "Miyoshino", were diploid (2n=22) with medium-height, slender stems, and medium-sized flowers. And two cultivars, Master Ingram and Paul Arnold, were triploid (2n=33) with higher-height, dick stems, and larger flowers. Furthermore, the above-described isolated strain was confirmed to be tetraploid (2n=44) and mediumed .as "Spontaneous Tetra" temporarily. This tetraploid showed several polyvalent chromosomes accompanied with many bivalent and univalent ones in the frst metaphases of meiotic divisions of PMCs, and then produced a plenty of fertile pollen grains which were distinctly larger than those of diploids. On the contrary, diploid "Miyoshino" showed 11 biva-ients, and triploid Paul Arnold produced ll biva-lents plus 11 univalents in the first metaphases, res pectively.

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The cause of early lethality of F₁ hybrid seedling induced by a crossing between Hatekunrnek (Korean cultivar.) and E.36 (Ethiopian cultivar.) was cytologically investigated. It was suggested that the early lethality occurring during germination of F₁ hybrid was caused through the inhibition of first DNA synthesis in the meristems of seed embryos.

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The rice plants at the different developmental stages of embryo were exposed to X-rays, the dose being 3, 6, 12 and 20 kR at any of the stages. The main results obtained were as follows : 1) The growth retardation in length and width of embryo by irradiation varied considerably with the development stage of embryo at the time of irradiation, the younger the embryo, the effects being the more severe. 2) The common cellular abnormalities observed after irradiation were the inhibition of nuclear division, extraordinary enlargement and vacuolation of cells, thickening of cell wall and degeneration of embryo. In general, these effects increased in degree and frequency with increasing radiation doses. 3) Especially when given irradiation dose was the higher dose, the observed vegetative prolifera-tiori accompanied by disruption of organized embryo were the more often. 4) The histological and morphological abnorma-lities produced by irradiation varied considerably depending on the stage of development of embryo at the time of irradiation, the most effective stage being the stage of the onset of differentiation in embryogenesis. 5) No definite conclusions as to the recovery process following irradiation could be drawn from the results of the experiments. However, it was supposed that in the case where irtadiations were carried out with the lower doses and at the later stages of embryogenesis, the cellular recovery seemed to occur the more predominantly. On the other hands, the replacement by division of the adjacent cells seemed to occulr the more predomina-ntly in the case where irradiation was applied with the higher dose and at the earlier stages of emb-ryogenesis.

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