1. With 2 varieties of paddy rice, the relation between day-length and heading time was investigated. The tillers in one plant were divided into two groupes. Long-day treatment applied to one groupe and short-day treatment applied to the other. The heading time became later in the former and earlier in the latter. 2. The effect of short-day condition was more remarkable in the treatment during 20 days from the stage of vegetative growth, but long-day condition was more effective in the case from the stage of reproductive growth. 3. It was presumed that the long-day and short-day inductions occurred independently in each tiller in one plant, but they somewhat canselled each other. 4. In order to experiment on the effect of photoperiod upon the heading of rice plant, the soft X-ray apparatus “Softex-J” was useful for observing the growth of young panicle from the time that the length became about 1～2cm.