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Acanthus ebracteatus

Acanthus ebracteatus flower.jpg
Acanthus ebracteatus susbp. ebracteatus flower, Singapore. This subsp. is known from only one location in Australia
Photo: John Yong

Common Names

Bractless Holly Mangrove [1], Purple Mangrove Holly [2]

Family

Acanthaceae

Name Reference

Acanthus ebracteatus Vahl

Origin of Name

‘Acantha’ means thorn or thistle (in Greek), and refers to the spiny leaves of some species. ‘E-bracteatus’ means without bracts (in Latin), and refers to the lack of bracteoles in this species [1].

Summary

Acanthus ebracteatus is typically a low viny herb that grows in under canopy patches. Absent or minute bracteoles on flowers and fruits. Two subspecies exist within Australia that do not appear to overlap in range. Acanthus ebracteatus subsp. ebarbatus is the most widespread and endemic to Australia, while Acanthus ebracteatus subsp. ebracteatus is known from only one location in Australia, the Escape River in Queensland [1].

Botanical Description

GROWTH FORM

Shrub or herb 1-3 m, non-woody, somewhat viny with sparsely branched stems; bark smooth, green; stem slender, 10 mm W, cylindrical, shiny-green with speckles, with or without axillary spines; roots occasional aerial along lower parts of reclining stems.

FOLIAGE

Leaves opposite, simple, narrowly ovate to oblong tending lanceolate, shiny green, 7.5-20 cm L, 2.5-5.5 cm W, glabrous, apex acute, base cuneate, margins either entire or spiny and dentate, presence of spines with greater sunlight and exposure; petiole 1-1.9 cm L.

REPRODUCTIVE PARTS

Inflorescence terminal, forming open bracteate erect spikes to 10 cm L, spikes extend with age; flowers in 4 ranks, to 20 pairs; flowers perfect, zygomorphic; bract 5 mm shorter than calyx, often caducous; lateral bracteoles absent; calyx 4-lobed, upper lobe conspicuous, enclosing flower bud, lower lobe smaller, lateral lobes narrow, wholly enclosed by upper and lower sepal; corolla white or deep purple, to 2 cm L, short tube closed by basal hairs; abaxial lip 3-lobed to entire, adaxial lobes absent; stamens 4, subequal with thick hairy connectives; anthers medifixed each with 2 cells aggregated around style; ovary bilocular with 2 superposed ovules in each loculus; style enclosed by stamens, capitate to pointed stigma exposed; fruit 4-seeded capsule, ovoid, green, shiny, smooth, 2 cm L, 1 cm W.

DISPERSAL PROPAGULE

Seeds orbicular, about 1 cm L, germination hypogeal; testa delicate, wrinkled whitish green; cotyledons flattened, green.

[1].

Distribution

Acanthus ebracteatus occurs in estuaries throughout the Asian tropics to northern Australia. In Australia, the species occurs in estuaries and embayments from Wyndham, King River, Western Australia (15° 30' S, 128° 05’ E) in the west, across the Northern Territory, to the Escape River, Queensland (10° 59’ S, 142° 40’ E) in the east [1].

Similar Species

The species is often confused with A. ilicifolius. They share most vegetative growth form characters, including: viny undercanopy thickets, their scrambling habit over adjacent vegetation, adventitious aerial roots, leaf form, stem spines, and spiny leaves. Reproductive characters, like pollination and seed release are similar also to A. ilicifolius. Acanthus ebracteatus is distinguished from A. ilicifolius by its flowers being white, slightly smaller (2-2.5 cm L), with absent or minute bracteoles, and smaller fruits (<2 cm L) [1].

Differentiation between these two species from foliage alone can be difficult, however, in general, A. ilicifolius has prickly leaves and stems with axillary spines and A. ebracteatus has non-prickly leaves and stems with no axillary spines. A. ilicifolius can posses non-prickly leaves, especially in conditions of low sunlight or strong growth. A. ebracteatus can occasionally possess prickly, toothed foliage, although no NT specimens have been observed with spines in the leaf axis ([2]).

For illustration of the above distinguishing characters see Mangrove Watch Australia: http://www.mangrovewatch.org.au/index.php?option=com_content&view=category&layout=blog&id=33&Itemid=300171

Habitat

High-mid intertidal, intermediate estuarine position [1].

Biology

In Australia, peak flowering occurs from September to December, and peak fruiting in January and February ([3], [2]).

Sunbirds Nectorinia jugularis have been seen visiting Acanthus spp. in Australia, although large bees (Xylocopa) are likely the main pollinator ([4]).

Ethnobotany

For ethnobotanical information see [2].

Key to two Acanthus ebracteatus subspecies of Australia

They are distinguished by flower colour, leaf shape, stem spines, and flower hairiness along upper suture.
1. Flowers deep purple, leaves mostly entire (rarely spiny), stems without spines, anthers not hairy on upper suture Acanthus ebracteatus subsp. ebarbatus
1*. Flowers white (maybe streaked red), leaves entire and/or spiny, stems often without spines, anthers hairy on upper suture Acanthus ebracteatus subsp. ebracteatus

Peter K. L. Ng and N. Sivasothi (editors) (2001). Jeruju hitam Acanthus ebracteatus. Guide to the Mangroves of Singapore. Available online: http://mangrove.nus.edu.sg/guidebooks/text/1045.htm

FloraBase, the Western Australian Flora: http://florabase.dec.wa.gov.au/browse/profile/7160

2161477520 1a2a1d49a8 b.jpg
Acanthus ebracteatus susbp. ebracteatus, Thailand.
Image: Tony Rodd
-- NormDuke and EmmaClifton - 2011-12-13 - 15:46

References

  1. Duke, N. (2011). Mangroves of Australia. Manuscript. Vers.: 27 Sept 2011. (more)
  2. Wightman, G. (2006). Mangroves of the Northern Territory, Australia: identification and traditional use. Northern Territory. Dept. of Natural Resources, Environment and the Arts, Palmerston. (more)
  3. Duke, N.C. (2006). Australia's Mangroves. The authoritative guide to Australia's mangrove plants. University of Queensland, Brisbane. (more)
  4. Primack, R.B., Duke, N.C. and Tomlinson, P.B. (1981). Floral morphology in relation to pollination ecology in five Queensland coastal plains plants. Austrobaileya 1(4): 19-62 (more)

Biological Interactions
Relation Taxon GroupSorted ascending
PollinatedBy Nectorinia_jugularis Birds
PollinatedBy Hymenoptera Insects
OccursWith Avicennia_integra Vascular_Plants