Hydnophytum

Hydnophytum is a genus of flowering plants in the family Rubiaceae that comprises several species known collectively as ant-plants (myrmecophytes). These plants form mutualistic associations with ants, which inhabit specialized chambers within a large, tuberous caudex. The ants provide protection from herbivores and access to nutrients through their waste, while the plant offers shelter, nesting space, and often nectar.

Native to tropical regions of Southeast Asia, New Guinea, and the western Pacific, Hydnophytum species are predominantly epiphytic, growing on tree trunks and branches in humid, shaded forests. They have simple, evergreen leaves arranged oppositely along slender stems and produce small, inconspicuous flowers. The fruit is typically a berry containing seeds.

Morphology distinguishes Hydnophytum by its prominent caudex, which is usually above or at ground level. The caudex contains a network of hollows and entry points that accommodate ant colonies. Flowers are generally small and may be white or pale, and the fruits are berries that disperse seeds.

In cultivation, Hydnophytum is valued by plant enthusiasts for its distinctive underground architecture. They require warm temperatures, high humidity, and indirect light. Many species are grown mounted or in well-draining media and are propagated by division of the caudex or by seed.

Systematics: Hydnophytum and Myrmecodia are two Rubiaceae genera known for myrmecophytism, with Hydnophytum typically presenting a smoother, more compact caudex compared to Myrmecodia. A notable species is Hydnophytum formicarium, commonly cultivated as an ant plant.