We have known for a long time that flowers of different plants open and close at different times of day. This is so    16  that there seems to be no need to ask the reason for it. Yet no one    17  understands why flowers open and close like this at particular times. The process is not as    18  as we might think, as recent experiments have shown. In one study, flowers were kept in constant    19 . We might expect that the flowers, without any information about the time of day, did not open as they    20  do.