Abstract: During 1955 to 1957, we investigated the characteristics of the Kiayuan free-seeded cotton (G, barbadense, L.) originated in the subtropic or tropic region, including the response to the day length, photoperiodic characteristics and the developmental conditions for the growth of young squares. The results obtained are as follows: 1. The normal completion of the photo-stage of Kiayuan free-seeded cotton requires short day. In the range of day length used in this experiment, 10, 12, 13, 16, 24 h., the normal day length of 10-hour illumination was the most favorable. Lengthening of the daily period of illumination retarded the initiation of squaring to a certain extent. 2. The photo-stage of this variety completed under 10 h. short day treatment 18–20 day after seedling emergence. Thereafter, as regard to their subsequent initiation of square meristems, they were already insensitive to the day length. 3. The requirement of short day conditions for completion of their photo-stage was not the same with different seedling ages. The older the age the shorter the days required for satisfying their photo-stage development. Most favorable seedling age for completing the effect of short day treatment found in this experiment was 10 days after their emergence. Hence, the photo-stage could be completed in 8–12 days under 10 h. short day treatment. After then, the period required for the growth of the induced bud meristems to emerge as squares was about 10 days. 4. The development of the individual plants delayed under high temperature conditions, and the most favorable seedling age for reacting with short day treatment came around 15 days after seedling emergence, but its requirement for short day treatment decreased to 6 days. 5. Although the squares could be emerged after the completion of their photo-stage induction, the short day condition is still necessary for their subsequent development. Under natural day, length in August in Nanking, the young squares wilted soon after their appearance. Only the plants which had grown under short day conditions until 5–7 days before blooming were moved to the natural day could bloom normally. 6. The phasic development of individual plant is in close relationship with the formation of organs. The formation of organs and its full development have their appropriated conditions respectively. For the induction of flowering of the Kiayuan free-seeded cotton, short day conditions were necessary. Squares could emerge and get continuous formation only on the basis of the fore-mentioned qualific changes. But a certain period of short day was still required for its subsequent development. The developmental process of the photo-stage could be delayed by high temperature, but there was no change in its requirement for short day. Based on these fundamental findings we are enabled to control the desired blooming time of this cotton type by adjusting the sowing date or applying short day treatment in proper seedling stage. It has its importance as known both in plant breeding and cultural practice.