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I think that Fitzgerald uses the flower imagery implicit in the female names Daisy and Myrtle to emphasis and enhance the character traits of the two women.
Daisy, the flower, has resonances of purity and innocence. The character also has these layers in her personality. Myrtle on the other hand has white blooms but dark berries and again, this underscores and adds to our understanding of the character. She may appear to be beautiful but there are elements of darkness present.