The plant called the scarlet gilia can have either red or white flowers. It had long been thought that hummingbirds, which forage by day, pollinate its red flowers and that hawk-moths, which forage at night, pollinate its white flowers. To try to show that this pattern of pollination by colors exists, scientists recently covered some scarlet gilia flowers only at night and others only by day: plants with red flowers covered at night became pollinated; plants with white flowers covered by day became pollinated. Which of the following, if true, would be additional evidence to suggest that hummingbirds are attracted to the red flowers and hawkmoths to the white flowers of the scarlet gilia?