THE ANT‐PLANTS MYRMECODIA AND HYDNOPHYTUM (RUBIACEAE), AND THE RELATIONSHIPS BETWEEN THEIR MORPHOLOGY, ANT OCCUPANTS, PHYSIOLOGY AND ECOLOGY
SUMMARY 
The history of investigation of the ant-plants is briefly reviewed and the external morphology of the plants described. Within Papua New Guinea ant-plants are abundant in open-canopied environments and less common but more diverse in rain forest. They also occur terrestrially above 2400 m. Initial tuber development was found to be similar that described by Treub (1883). Later cavities are formed by successive, increasingly complex phellogens. In certain Myrmecodia spp. later cavities are differentiated into regions with distinct surface characteristics, shapes and positions. Cavities in Hydnophytum are simpler in shape but sometimes differentiated in other ways. The anatomy of the cavity surfaces, especially the putative absorption sites ('warts') is described. 
 
The ant Iridomyrmex cordatus, which is common in ant-plants in other regions, is found in ant-plants in open habitats in Papua New Guinea, but is replaced by I. cf. scrutator in rain forest and above 2000 m. Two fungi, known also from ant-plants in Java (Miehe, 1911a), occur in the cavities of ant-plants throughout Papua New Guinea. Arthrocladium sp. Papend. grows on cavity surfaces where ant faecal material is present and the other (an unidentified Monilialine fungus) is parasitic on smooth, clean surfaces. The former is most common and the latter almost confined to Myrmecodia spp. occupied by Iridomyrmex cordatus. 
 
Experiments suggested that both Hydnophytum under artificial conditions and Myrmecodia under field conditions grow better in the presence than absence of Iridomyrmex cordatus and that the stimulus might be that of minerals supplied by the ants. Radioisotopes in both organic and inorganic compounds fed to I. cordatus were deposited preferentially by the ants on the warted cavity surfaces where ant faecal material was present, and were absorbed by the plants. In the discussion it is argued that the plant/ant relationship has had a considerable influence on both the ecology and evolution of the plants, but that this varies in different species.