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Definition of FOUR-O'CLOCK
: any of a genus (Mirabilis) of chiefly American annual or perennial herbs (family Nyctaginaceae, the four-o'clock family) having apetalous flowers with a showy involucre simulating a calyx; especially: a garden plant (M. jalapa) with fragrant yellow, red, or white flowers opening late in the afternoon
Ornamental perennial plant (Mirabilis jalapa; family Nyctaginaceae), also called marvel-of-Peru or beauty-of-the-night, native to the tropical New World. It is a quick-growing species up to 3 ft (1 m) tall, with oval leaves on short leafstalks. The stems are swollen at the joints. The plant is called four-o'clock because its flowers, which vary from white and yellow to shades of pink and red, sometimes streaked and mottled, open in late afternoon (and close by morning).