A mosaic of evolutionarily ancient and isolated pockets such as the Guayana Highland of northern South America can be expected to house unusual organisms, and the three genera that comprise this treatment are among the oddities. The Guayana Highland is composed of the famous tepuis (flat-topped mountains with nearly vertical sides harboring cool, humid forest) separated by lowland tropical forests or savanna. Gleasonia, Henriquezia, and Platycarpum are distributed across the Guayana Highland, also frequently in the drainages of some major rivers leading away from it. These trees and shrubs are conspicuous by having showy, usually zygomorphic, flowers arranged in complex terminal cymes, large valvate stipules, woody clam-shaped capsules, and flat, papillose seeds. They often grow in high concentrations. The treatment includes keys, descriptions, distributions maps, black and white illustrations, and photographic images.