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vert bar A revision of the ant-plant genus Hydnophytum (Rubiaceae)

Matthew Jebb (National Botanic Gardens) Camilla Huxley (Oxford University)
Last updated: 8th February 2009

Project Background

Hydnophytum is the largest of the five genera of 'ant-plants' in the Rubiaceae. These plants are almost exclusively epiphytic, and develop chambered tubers at the base of their stems, which act as homes to ants or other creatures - a remarkable symbiotic relationship in which the ant colony gets a home, and leaves behind detritus that helps to feed the plant.

The other four genera (Myrmecodia, Myrmephytum, Squamellaria and Anthorrhiza) have been revised (Huxley & Jebb, 1991b, 1991c and 1993; Jebb, 1991b, 1993). Of the 108 species of Hydnophytum that have been published, 33 have been retained in this revision, and a further 22 species are described as new. Eleven species remain known from their holotype alone, a problem with many New Guinea centred genera. Six species are inadequately known, either through loss of the type, or paucity of material, and these are retained as ‘little known species’. Five names are excluded from the genus.

Hydnophytum kajewskii, a species of ant-plant from Bougainville island Fifty-five species are therefore recognised, of which 22 are described as new. Forty-four species are found in New Guinea, or its offshore islands, and of these, 42 are endemic. Two species are found in the Fijian islands, one in Vanuatu, three in the Solomon islands, five in Indonesia, and two in Australia. The Royal Society of London gave a grant towards preparing drawings of all the species, and these were done by Rosemary Wise of Oxford.

Hydnophytum kajewskii (right), a species of ant-plant from Bougainville island, has the most elaborate tuber structure known, which must rank as one of the most elaborate and bizarre vegetative structures in the entire plant kingdom.

The boat-shaped tuber which grows up to 25cm in length, and 12cm wide. The tuber cavities each open by a pair of large, lipped entrance holes which are aligned along the upper edge of the tuber. The tuber may contain ants, or a range of other invertebrates, from spiders, scorpions, cockroaches and myriopods. New cavities are added one at a time to the apex of the tuber.

The diversity of tuber structure within Hydnophytum is in marked contrast to that seen in the other genera, and suggests that while the other genera are certainly monophyletic, Hydnophytum is a paraphyletic grouping (Jebb 1991a).




References

Huxley, C.R. & M.H.P. Jebb. 1990. New taxa in the myrmecophilous Psychotrieae (Rubiaceae). Bull. Jard. Bot. Nat. Belg. 60: 420--421.
Huxley, C.R. & M.H.P. Jebb. 1991a. The tuberous epiphytes of the Rubiaceae 1: A new subtribe - The Hydnophytinae. Blumea 36: 1-- 20.
Huxley, C.R. & M.H.P. Jebb. 1991b. The tuberous epiphytes of the Rubiaceae 2: The new genus Anthorrhiza. Blumea 36: 21--41.
Huxley, C.R. & M.H.P. Jebb. 1991c. The tuberous epiphytes of the Rubiaceae 3: A revision of Myrmephytum to include Myrmedoma. Blumea 36: 43--52.
Huxley, C.R. & M.H.P. Jebb. 1993. The tuberous epiphytes of the Rubiaceae 5: A revision of Myrmecodia. Blumea 37:271--334.
Jebb, M.H.P. 1991a. Cavity structure and function in the tuberous Rubiaceae. In C.R.Huxley & D.F.Cutler, Ant-plant Interactions: 374 --389. Oxford Univ. Press.
Jebb, M.H.P. 1991b. The tuberous epiphytes of the Rubiaceae 4: A revision of Squamellaria. Blumea 36: 53--61.
Jebb, M.H.P. 1993. Anthorrhiza camilla -- A new species of Rubiaceous ant--plant. Blumea 37: 341--344.