Extrafloral nectaries in an Australian rainforest: structure and distribution

Extrafloral nectaries (EFNs) are of ecological, evolutionary and taxonomic importance in many plants, but are often overlooked in botanical descriptions and have rarely been studied in humid Australian forests. We examined EFNs in a tropical rainforest in northern Queensland, Australia. A total of 29 plant species was found bearing EFNs within the 1-ha study plot at the Australian Canopy Crane Project and an additional 10 EFN species were found in rainforests and other habitats outside, but nearby, the plot. The records include 12 genera in which EFNs have not been previously reported (Ardisia, Bambusa, Castanospermum, Dysoxylum, Melicope, Flagellaria, Glochidion, Ichnocarpus, Merremia, Rockinghamia, Syzygium, Wrightia), including one new family (Flagellariaceae). In the study plot, 13 tree species (17% of tree species with dbh >10 cm), 10 climbing plant species (21%) and six shrubs had EFNs, a similar proportion compared with tropical forests on other continents. Morphology of most EFNs was studied by using scanning electron and light microscopy. Extrafloral nectaries were assigned to five different structural types (sensu Zimmermann 1932): flattened, elevated, pit, scale-like and formless nectaries. EFNs from all species were regularly visited by ants, allowing detection of many otherwise inconspicuous nectaries.