34 THE FLOWERING PROCESS 



twilight at various times. Atsushi Takimoto and Katsuhiko Ikeda (74) 

 in Japan have performed a number of experiments of this type. With 

 the Japanese morning glory they found that plants left in the twilight 

 after it had dropped to an intensity of about 10 to 20 ft-c flowered 

 as well as plants which were placed in complete darkness at this time. 

 In the morning, however, plants were inhibited in their flowering if 

 they were moved from darkness to twilight any time after the light 

 intensity had reached 0. 1 ft-c. Thus they concluded that the biological 

 night for this plant begins in the evening at about the beginning of 

 astronomical sunset and ends in the morning at the time of the 

 beginning of civil twilight. The principle was applied to other plants, 

 and it was found that sensitivity in the morning or the evening varies 

 considerably from species to species. Thus Oryza sativa is relatively 

 insensitive both times, and the biological day-length about corres- 

 ponds to astronomical day-length. Soybean and Perilla acted 

 somewhat like Japanese morning glory, but cocklebur was very 

 sensitive in the evening (0.1 to 1.0 ft-c) and somewhat less so in the 

 morning (1.0 to 5.0 ft-c). 



The effect of clouds during twilight could be quite complicated. 

 If it is merely a matter of reaching a certain critical light intensity, 

 then we could easily state the nature of the complication. Thus in 

 Japanese morning glory the 20 ft-c level might be reached up to 

 30 min sooner on the evening of a cloudy day, but the 0. 1 ft-c level 

 is probably reached at about the same time (5 to 10 min later when 

 cloudy, perhaps) on either a cloudy or a clear morning. The problem 

 is, it is not the time of critical intensity which we need to know, but 

 the time of the critical amount of F-phytochrome. This problem has 

 yet to be studied, but it should be fairly apparent that clouds during 

 sunrise or sunset could indeed influence flowering to a certain extent, 

 and this might readily account for the observation that many plants 

 sensitive to photoperiod do not always flower at exactly the same 

 time each year. Of course, plants that require many cycles for 

 induction would tend to average out the weather anyway. 



An interesting sidelight of the Japanese work was the finding that 

 Japanese morning glory, although it is an absolute short-day plant, 

 is induced to flower in the open in Japan near the end of June when 

 the days are longest. Its critical dark period is shorter than the 

 shortest night at that latitude. It apparently fails to flower in response 



