About 30 genera and 300 species of herbs, shrubs, and trees of wide geographical distribution in warm regions but most abundant in America; a few grown for ornament. Leaves simple, opposite or alternate, entire, exstipulate; inflorescence various, flowers regular, bisexual or rarely unisexual, usually subtended by an involucre or separate or united bracts; petals 0; calyx inferior, often petaloid, campanulate, tubular or slaverform, persistent after flowering and enveloping the fruit, often woody or leathery; stamens 1 to many, hypogynous, free or united at base; style 1; ovary 1-celled, 1-ovuled, sessile or stipitate; fruit a ribbed, grooved or winged achene. (Bailey 357).
The brilliance of our garden Bougainvillea, also in this family, must be attributed to sepals and bracts, not petals. It is probably best to enjoy the colorfullness of all of the Four O'clocks without even trying to figure out the difference between petals, sepals and bracts as they appear in Nyctaginaceae. (Dale 138).