Hydnophytum formicarum is an epiphytic myrmecophyte subshrub with succulent tubers associated in its growth with certain species of ants. The tuber contains within a complex labyrinth of chambers inhabited by ants colonies, with entrance holes for ants over the surface. As soon as the young plants develop a stem, the ants gnaw at the base of this, and the irritation produced causes the stem to swell; the ants continuing to irritate and excavate the swelling, it assumes a globular form, and may become even larger than a man's head. The walls of these chambers and the whole mass of the inflated stem retain their vitality and thrive, continuing to increase in size with growth. It appears that this curious gall-like tumour on the stem has become a normal condition of the plants, which cannot thrive without the ants. The ants provide the plant with a source of macronutrients through their debris. Ants likewise provide defense for the plant and prevent tissue damage, swarming to defend their home if disturbed.
