Ophelia used flowers as symbols of her deep sorrow and grief. She was very upset because her father has just been killed by Hamlet. Being a sensitive and intelligent young woman, Ophelia needed to express herself. In Hamlet, she does so by passing out flowers to the court in her seeming mad state of mind. Doing this she accused the King and Queen of the guilt she wanted them to feel. Using flowers in this indirect way is safer than directly stating her opinions before the Court.
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