Hybrids between distantly related verieties of rice were propagated in bulk or in pedigrees without artificial selection, and a number of lines were taken at random from the populations in F₆, . Variations among those lines were then investigated regarding various characters, i. e. (1) Potassium chlorate resistance, (2) Low temperature resistance, (3) Drought resistance, (4) Number of days of germination, (5) Weight ior shedding a grain, (6) Lengt, h-width ratio of grain, (7) Length of apiculus hair, (8) length of a.wn, (.9) Index-number of the degree of endosperm destnrction in alkali-test, (lO) Phenol-reaction, and several agronomic characters.

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F₇ or F₈ Iines taken at random from populations of hybrids between distant varieties (the same materials as reported in the 16-th contribution of, this series) were crossed with the parental varieties or a certain variety ' chosen as tester, and the variation among those lines in reaction-type of hybrid sterility was investigated. Various reaction-types were found, and it vJas pointed out that most of lines had reaction-types like one or the other parent, and that lines producing fertile hybrids with one parent tended to produce semi-sterile hybrids with the other. This may imply tb.at a number of sets of "GameticDevelopment genes" are present in the parental varieties, and they are linked with one anotber. Lines similar to one parent in the reaction-type of hybrid sterility tended to have characteristics similar to those of parent. This may be due to the linkage of "G.D. genes" with genes responsible for those chal acters. It was found furth ?? r that lines similar to one parent in the reaction-type of hybrid sterility or in the tender.Lcy of character combination tend to show a hybrid vigor in crosses with the other parerLt. It was discussed that the restriction of recombinations due to the "G. D. genes" might work as an isolating mechanism between distantly related varieties of rice.

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To clarify the relationship of short-day genes with the flowering time in cultivated varieties of Nicotiana tabaculee, the genetic analysis in a cross between Xanthi and Bright Yellow was carried out in this study. The former variety is an intermediate type and the latter is, a short-day type. The number of leaves was used to analyse the genes affecting the short day response because of the close correlation between bud formation and the number of leaves. Furthermore, the distribution of leaves for the segregated populations were so contmuous that the "partitronmg method" was best fitted to be applied. The theoretical means and distribution for each population were calculated with the assumption that three genes were involved and there was no genic interaction. These tbeoretical values fitted well into the observed ones.

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The present paper deals with the analytical study on the genes controlling resistance to Ph.ytophthora ilefestales in potato tubers. It was shown in the former report by the author(1957) that an almost complete correlation exists between leaf and Ituber resistance, providing reliable basis for estimating leaf resistance by means of tuber trial. Strains of Phytaphthora ilufestans used were three in number; common strain H1(race 0)and two specialized strains, H₂ (race4)* and H₃ (race1)*. There are a number of hybrid potatoes (S. demissum x S. tuberosum) in Japan which have been bred since 1939, some of which were employed for the experiment. ' Tubers were cut in halves and a concentrated sporangial suspensio:n of the fungus was sprayed over the cut surface. There were five types of reactions classified according to the different necrotic responses (Fig. 2). As a result of inoculation test, it was revealed that all of the hybrid potatoes thnat were bred before 1956 possessed two genes, Ra and Rb for resistance singly or in combination. These two genes were recognized to follow the same pattern, independent and dominant, as was shown by many other workers. The most interesting fact is that the phenotypic expression of Rb (R4)* is of weaker type (represented by type f in Fig. 2) than that of Ra (R1)*, presenting comparatively so rich growth of aerial mycelium that sometimes one might fail to detect t:he existence of gene Rb (R₄). The intervention of some modifiers in the expression of each of the above twb genes has been well surmised and these modifiers are expected to exist in S. tuberosum as well as in S. demissum.

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A comparative study was made on the photoperiodic response using the following 10 varieties of tobacco differing in earliness and lateness : Nambu (very early) ; Matsukawa (early) ; Bright Yellow, Yellow Special-A and Xanthi (medium early) ; White Burley, Kokubu and Oxford 26 (late), and Talcehara-tayo and Kochi-tayo (very la:te). These varieties were subjected to five different day-1ength, namely, 6, 8, 10, 12 and 14 hours. The experiments were conducted during summer months (June to September) at high temperature (23-30 'C) and also during winter (November to February) at low temperature (10-20 'C). The results obtained may be summarized as follows : 1. The earliest flower-bud initiation occured under 8-hours day in all of the varieties tested both at high and low temperatures. Reproductive growth tended to be delayed more and inore as the photoperiod was prolonged. As the retardation of flowering became more marked at the high temperature than at the low temperature, varietal differences in' this nature broadened more and more. 2. Differences in the responses to photoperiod and number of the total leaves of plants of these varieties tested at the end of the experiments enabled us to classify them into three groups, namely, (1) highly sensitive to longer photoperiod, (2) moderately sensitive and (3) slightly sensitive. Furthermore it was also recognized that most of the very late varieties belonged to the group, (1), medium to late varieties to the group (2) and early to medium varieties to the group. (3). However, there was a marked intravarietal difference in number of total leaves, the reason of which remained unknown.

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As written in the other paper, Iow temperature accerelates flower initiation and declreases the number of total leaves. In this paper the effect of temperature on days of fiower initiation and number of total leaves, with special reference to its 'varietal differelrce, are dealt with 16 varieties selected from the Japanese tobacco varieties for fluecuring and those for air-curing were used as material. As shown in table 2, they were Tepeatedly planted on four different dates at about 30 days intervals from April to August in 1955, and were subjected to 8-hburs day. It is because under 8-hours day most of the tobacco varieties have been known to show the least difference in their photoperiodic responses.

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Pollen grains Which had been treated with various dcsages of X-rays, ranging from 0 to 9600 r, were dusted on the stigma of various species in order to reveal the infiuence of X-rays upon interspecific crosses in Nicotialea. Reciprocal crosses of N. tabacum (n=24) with N. alata (n=9). N. Lalegsdorffii (n=9) and N. Iogiflora (n=10) were used as materials. In the case of crosses N. tabacum ( ) x alata group ( ), the majority of flowers set capsules containing many empty seeds, if fresh pollens were used. With increasing X-ray dosage, filled seeds were produced attaining the maximum (2-8 %) at 4800-9600 r. Beyond that amount, their frequency was again decreasing. Percentage of capsules, number of seeds per capsule, weight of single seed etc, displayed a similar trend. Most suitable dosages for germination, however, are lower than those for producing filled seeds.

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Although the production of seedless triploid, even tetraploid. F1 hybrid watermelons is becoming popular year by year in Japanese gardening, they are not yet used for commercial products in the United States and other neighboring countries where many a strain of watermelon are grown in large or small scales. It has been sh.own, however, as a result of several experimental tests that the present Japhnese seedless watermelons are not always so fitted for growing in America as expected by Japanese seedmen, because of the weak germinating capacity of seeds, rather smaller and earlier ripening quality as compared to the ordinary American varieties, and also too brittle ri[nd and flavorless flesh of the fruits to market.

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1. The germination of F₃ from the cross, Triticum cowrpactum ( ) x Secale cereale ( ), the external characteristics, the fertility and the somatic chromosomes of the plants, grown therefrom were investigated. 2. The percentage of germination of seed from each F₂ plant was varied, ranging from 0 to 100 (Table 1). 3. Of the total 151 F₃ plants obtained the author was able to test only 81 for their somatic number of chromosomes, the remaining 70 having failed to offer preparations, whose number varied from 44 to 56 (Table 2). The somatic number of the F₃ plants from the F₂'s with 47 and 48 chromosomes seems to show a tendency to decrease and approach to 42, whereas that of individuals from the F₂'s with chromosorrie numbers of rLear by to 56 increase and approach to 56 as the generation proceeds. They are expected to become stable and constant when their number reaches 42 and 56 respectively. 4. 88 (ca. 5%) of the 151 plants proved fertile, of which 53 were exarnined as to the, degree of fertility (Table 4).

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