Nectar - Extrafloral Nectaries

Extrafloral Nectaries

See also: Myrmecophily and Plant defenses against herbivory

Extrafloral nectaries (also known as extranuptial nectaries) are nectar-secreting plant glands that develop outside of flowers and are not involved in pollination. They are highly diverse in form, location, size, and mechanism. They have been described in virtually all above-ground plant parts—including leaves (in which case they are known as foliar nectaries), petioles, stipules, cotyledons, fruits, and stems, among others. They range from single-celled trichomes to complex cup-like structures that may or may not be vascularized.

Extrafloral nectaries on a red stinkwood (Prunus africana) leaf

In contrast to floral nectaries, nectar produced outside the flower generally have a defensive function. The nectar attract predatory insects who will eat both the nectar and any plant-eating insects around, thus functioning as 'bodyguards'. Foraging predatory insects show a preference for plants with extrafloral nectaries, particularly some species of ants and wasps which have been observed to directly defend the plants. Among passion flowers, for example, extrafloral nectaries prevent herbivores by attracting ants and deterring two species of butterflies from laying eggs. In many carnivorous plants, extrafloral nectaries are also used to attract insect prey.

Extrafloral nectaries were originally believed to simply be excretory in nature (hydathodes). Their defensive functions were first recognized by the Italian botanist Federico Delpino in his important monograph Funzione mirmecofila nel regno vegetale (1886). Delpino's study was inspired by a disagreement between him and Charles Darwin with whom he corresponded with regularly. Darwin believed that extrafloral nectaries were simply hydathodes, while Delpino believed they had a defensive function, especially among myrmecophilic plants.

Extrafloral nectaries has been reported in over 3941 species of vascular plants belonging to 745 genera and 108 families. 99.7% of which belong to flowering plants (angiosperms), comprising 1.0 to 1.8% of all known species. They are most common among eudicots, occurring in 3642 species (of 654 genera and 89 families), particularly among rosids which comprise more than half of the known occurrences. The families showing the most number of recorded occurrences of extrafloral nectaries are Fabaceae, with 1069 species; Passifloraceae, with 438 species; and Malvaceae, with 301 species. The genera with the most number of recorded occurrences of extrafloral nectaries are Passiflora (322 species, Passifloraceae), Inga (294 species, Fabaceae), and Acacia (204 species, Fabaceae). Other genera with extrafloral nectaries include Salix (Salicaceae), Prunus (Rosaceae) and Gossypium (Malvaceae).

Foliar nectaries have also been observed in 39 species of ferns belonging to 7 genera and 4 families of Cyatheales and Polypodiales. They are absent, however, in bryophytes, gymnosperms, early angiosperms, magnoliids, and members of Apiales among the eudicots. Phylogenetic studies and the wide distribution of extrafloral nectaries among vascular plants point to multiple independent evolutionary origins of extrafloral nectaries in at least 457 independent lineages.