The Aerial Life of the Ant Plants Hydnophytum and Myrmecodia

There is a group of plants, commonly known as ant plants that establish a very unusual relationship with ants, but they do not use these insects as pollinators. These plants from the genera Hydnophytum and Myrmecodia are all epiphyts, i.e. they live on branches and trunks of trees from where they get mechanical support and water derived from moisture from the air and rain and sometimes from debris accumulating around it. They are all found in mangroves of tropical forests of northern Australia and Southeast Asia, mainly in the Indonesian archipelago. The ant plants are small plants that store water and food in a swollen stem, called caudex that grows spines over time in some species of both genera. Standing high above the soil and with no roots on it to absorb nutrients the ant plants developed an unusual way of getting them by hosting special guests, ants. The caudecis can grow as big as a football ball and have numerous holes and interconnected chambers and tunnels inside them where ants live. There inside, the queen ant lays its eggs continuously and the larvae are fed and protected in special chambers. In other different and darker chambers ants lay their excrements and debris from the insects they hunt down. These nitrate and phosphate rich organic wastes will then become the nutrient supply that the ant plant needs. The walls of these chambers and the interconnecting tunnels are lined with glands in which specialized cells absorb nutrients from the food wastes accumulated by the ants. In this way, the ants supply food for the plant and in return they obtain shelter and a place to establish their own colony. The spiny caudex offers protection for the ants against predators and the ants also provide defense for the ant plant and prevent its tissue damage. This especial mutualistic relationship between the ant plants and ants is called myrmecotrophy, and the ant plant a myrmecophyt. This symbiosis and the specialization observed in the caudex of the ant plants allows these plants to effectively obtain nutrients (via the ants) from a much larger area than the roots could ever cover, and survive in such a harsh and difficult environment that characterizes the mangrove forests. Some species of the genus Hydnophytum can be grown as house plants without the ants being present. This shows that theses species do not have an obligate mutualistic relationship with ants in order to survive. However, the conditions which allow growing them as houseplants are most likely not met in nature in their natural habitat. This mutualistic relationship between ants and the ant plants differs from that of the bullhorn acacia, in Central America, in that the ant plant does feed the ants but serves only as place for the ants to establish their colony.