the quassia family of flowering plants, in the order Sapindales, comprising 25 genera of pantropical trees, including Ailanthus, or the tree of heaven (q.v.). Members of the family have leaves that alternate along the stem and are composed of a number of leaflets arranged along an axis. Most species have small flowers, bitter bark, and fleshy fruits that are sometimes winged. The tree of heaven is often planted as an ornamental along city streets because it is smoke- and insect-resistant. Female plants are preferred because the male flowers release a disagreeable odour. Several varieties have colourful, twisted fruits and coloured leafstalks. Bark of species of the genera Quassia and Picrasma yields quassia, a bitter substance used in medicines. The crucifixion thorn (Castela emoryi) is native to the deserts of the southwestern United States.