The purple and white Passiflora nelsonii, which grows on vines high up in the trees in rain forests in southern Mexico and Guatemala, had never bloomed in captivity.
Experts say someone plucked one of the rare passionflowers in Chiapas, Mexico in 1895 and pressed it between sheets of paper. It soon turned brown and the person who picked it never said what color it was.
That specimen is at the Smithsonian Institution in Washington.
"It's exciting to finally see what this flower looks like," said John MacDougal, formerly conservatory manager at the Missouri Botanical Gardens in St. Louis. "I've been teased and tortured by that brown specimen at the Smithsonian for so many years."
Butterfly World had the specimen nearly two decades before it bloomed. Five buds were found two weeks ago. Two fell off, two flowered and one is expected to flower within a few days.
The flowers live about a day.