Abstract
In my preceding report (SAMATA 1959 b) it was noted that the expession of MS and MD genes which modify the flower-types in cosmos is subject to the influence of temperature to a considerable degree. The present experiment was designed to determine the sensitive period to the cumulative effect of temperature of these genes. Fourteen stra:ins with known genotypes were selected as materials for this experiment. Each strain was propagated by cutting and many individuals belonging to the same clone were grown. All individuals were divided into five groups in such a way that each group was sure to include one individual of each strain. After the interval of 12 days, four of these groups were removed, one by one, into the greenhouse kept at 30°C constant temperature where they were left until flowering. By this method they were subject to high temperature treatment at different stages of flower development. One group was left out of doors all the time, and was used as the control. When the plants came into bloom, each flower type of the treated groups was compared with th.at of the control of the same clone, and the changes of flower-types between them were examined with the naked eye and by comparing the photographs taken at different periods of fiowering. By this experiment it was made clear that the changes of both ray and disk florets take place only when the plants have been placed at least l0-15 days before flowering in the room kept at 30°C. From this it may be asserted that the sensitive period to the cumulative effect of temperature of MS or MD genes is l0-15 days before anthesis. To ascertain this period, another experiment wascarried out in which several plants with different flower-types growing in the field were transplanted into pots at the flowering time and placed in the room kept at 30°C. After 14 days an examination of the flower-types revealed that those with Ms or MD were changed to such types as had been expected. The third experiment was made to examine the cumulative effect of low-temperature in the case of MD gene. Two of the three plants which prdduced the single type of flower-head during the hightemperature season were placed in a room with lO-13°C constant temperature and it was found that they change from single to double flower-type after 30 to 45 days in the room, showing that, the sensitive period to low-temperature of MD gene is within a certain period of time after flower-initiation.
