The Secret Language of Flowers
Floriography or the language of flowers was popular in the Victorian era. Flower dictionaries where each flower is given a particular meaning were widely used. Mythology, folklore, and legends explained how flowers obtained their symbolic meanings. By decoding the meanings of the flowers in a bouquet the Victorians could send secret messages of love and friendship.
Some flowers convey rather negative meanings, so I suppose it was possible to send those messages as well. Two books I've enjoyed on the subject are The Secret Language of Flowers by Samantha Gray and A Victorian Flower Dictionary by Mandy Kirkby with an introduction by Vanessa Diffenbaugh.
Vanessa Diffenbaugh wrote The Language of Flowers in which her main character communicates through the language of flowers.
I love lily of the valley which is symbolic of the return of happiness. Catherine, Duchess of Cambridge carried it in her bouquet designed by Shane Connolly. This bouquet also included the flower sweet william, symbolic of gallantry and stephanotis, symbolic of eternal happiness.