It is well known that in Zinnia elegans the frequency of fiower heads exhibiting doubleness in a population decreases fast in a few years, when seeds are taken continuously in open-pollinated condition. The object of the present study was to investigate the cause of the phenomenon above mentioned. from the genetic standpoint. The flower heads of Zinnia are highly variable in respect to the number of petalous and tubular florets in a 1lead. In 1950, a reciprocal cross was carried out in the variety Pumila scarlet between completely double, No. 4-5, and completely single, No. 4-1. The flower types of the parents are presented in Fig. l. As Fig. 2 shows, eleven types were recognized in 1950 and 1951. The F1 hybrids between completely double (B type) and completely single (A type) showed the so-called E type, in which the basal part of the head consists of 2-3 Iayers of petalous florets, while the tubular fiorets occupy the remaining part, except for the top which has a few petalous florets. Between the reciprocally produced F1 hybrids no difference was observed. Those flower types were so easily affected by seasonal factors that one could easily distinguish between the summer and the autumn-heads of the F1 generation (see Fig. 3). According to the flower types shown in Fig. 2, 208 plants of the F2 generation were classified. From the results shown in Ta, b. 1, it may be concluded that the difference between the A- and the B- type is governed by three Mendelian genes. Thus, the complete double type would be recovered in the F2 population at the ratio of ohe per sixty four. Moreover, the completely double type yields only a few seeds. The completely single type and the approximating subtypes reproduce powerfully in comparison with the completely double type and its subtypes, because they have many tubular florets, and thus, give many seeds per plant. If the pollination by insects were at random, a general degeneration of an open-pollinated Zinnia population would inevitably take place. The role of F1 hybrids between the completely double or approximately double types and completely or apploximately single types affecting the genotypic constitution of successive generation was discussed.
