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
hawkmoths, 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.
What exactly is the meaning of covered here? Like if some flower is covered than how can it be pollinated?