Banksia
From Freepedia
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| Image:Banksia flower.jpg Banksia ericifolia at the Melbourne Botanical Gardens | ||||||||||||
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| About 80 species; see List of Banksia species | ||||||||||||
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Banksia subg. Banksia | ||||||||||||
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Banksia is a genus of around 80 species in the plant family Proteaceae. They are native to Australia, occurring in all but the most arid areas. Easily recognised by their characteristic flower spikes and fruiting "cones", Banksia are a well-known Australian wildflower and a popular garden plant. They grow in forms varying from prostrate woody shrubs to trees up to 25 metres tall. They are commonly known as Banksias or Australian Honeysuckle Trees.
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Description
Banksias grow as trees or woody shrubs. The largest trees, the Coast Banksia, B. integrifolia, and the River Banksia, B. seminuda, often grow over 15 metres tall, and may be up to 25 metres tall. Banksia species that grow as shrubs are usually erect, but there are several species that are prostrate, with branches that grow on or below the soil.
The leaves of Banksia vary greatly between species. Sizes vary from the narrow, 1–1½ centimetre long leaves of the Heath-leaved Banksia, B. ericifolia, to the very large leaves of the Bull Banksia, B. grandis, which may be up to 45 centimetres long. The leaves of most species have serrated edges, but a few, such as B. integrifolia, do not. Leaves are usually arranged along the branches in irregular spirals, but in some species they are crowded together in whorls.
Banksias are most easily recognised by their characteristic flower spike, and the woody fruiting structures that appear after flowering. The flower spike consists of a central woody axis with a furry coating; it is generally held erect, but hangs down in a few species. This axis is covered in tightly-packed pairs of flowers, which are attached to the axis at right angles. A single flower spike may contain over a thousand flowers.
Banksia flowers are usually a shade of yellow, but orange, red and pink flowers also occur. The colour of the flowers is determined by the colour of the perianth parts and often the style. The style is much longer than the perianth, and is initially trapped by the upper perianth parts. These are gradually released over a period of days, either from top to bottom or from bottom to top. When the styles and perianth parts are different colours, the visual effect is of a colour change sweeping along the spike.
As the flower spikes age, the flower parts dry up and turn a dark brown colour. In some species, old flower parts are lost, revealing the furry axis; in others, the old flower parts may persist for many years, giving the spikes a hairy appearance. Old flower spikes are commonly referred to as "cones", although strictly speaking they are not: cones only occur in conifers and cycads.
Despite the large size of the flower spike and the huge number of flowers per spike, only a very small number of flowers ever develop fruit, and often a flower spike will set no fruit at all. The fruit of Banksia is a woody follicle embedded in the "cone". These consist of two horizontal valves that tightly enclose the seeds. The follicle opens to release the seed by splitting along the suture, and in some species each valve splits too. In some species the follicles open as soon as the seed is mature, but in most species most follicles open only after stimulated to do so by bushfire. Each follicle usually contains two small seeds, each with a wedge-shaped papery wing that causes it to spin as it falls to the ground.
Distribution and Habitat
All but one of the Banksia species are endemic to Australia. The exception is the Tropical Banksia, B. dentata, which occurs throughout northern Australia, and on islands to the north including New Guinea and the Aru Islands. The other species occur in two distinct geographical regions: southwest Western Australia and eastern Australia. Southwest Western Australia is the main centre of biodiversity; over three quarters of Banksia species occur only there. Eastern Australia has far fewer species, but these include some of best known and most widely distributed species, including the Coast Banksia, B. integrifolia, and the Silver Banksia, B. marginata.
The vast majority of Banksia grow best in sandy soil, and the remainder, bar one, prefer rocky soil. The River Banksia, B. seminuda is exceptional for its preference for rich loams along watercourses. Most occur in heathlands or low woodlands, but B. seminuda and B. integrifolia both grow in forests. Most species do not grow well near the coast, and only one recently published species, B. rosserae, occurs in arid areas. Most of the eastern Australian species survive in uplands, but of the Western Australian species only the Stirling Range Banksia, B. solandri, survives at high altitudes.
Ecological aspects
Banksias are heavy producers of nectar, and so are important sources of food for nectariferous animals, including honeyeaters and small mammals such as honey possums, pygmy possums and bats . Many of these animals play a role in pollination of Banksia.
Other associated fauna include the larvae of moths and weevils, which burrow into the "cones" to eat the seeds and pupate in the follicles; and birds such as cockatoos, who break off the "cones" to eat both the seeds and the insect larvae.
One of the main threats to Banksia plants is bushfire. About half of Banksia species are killed by bushfire, but regenerate quickly since fire also stimulates germination of seed. The other species usually survive bushfire, either because they have very thick bark that protects the trunk from fire, or because they have lignotubers from which they can resprout after fire. In both cases, infrequent bushfires present no threat, but frequent bushfires can seriously reduce populations by kill seedlings before they reach fruiting age.
Another threat to Banksia is the algae (once thought to be a fungus) Phytophthora cinnamomi, commonly known as "dieback", which causes root-rot. Banksia's proteoid roots, which help it to survive in low-nutrient soils, also make it highly susceptible to this disease.
A number of Banksia species are considered rare or endangered. These include:
- B. brownii: endangered
- B. cuneata: endangered
- B. goodii: endangered
- B. oligantha: endangered; known from only one location
- B. plagiocarpa: endangered; known from only three small populations
- B. sphaerocarpa var. dolichostyla: endangered
- B. tricuspis: endangered
- B. verticillata: endangered; about 10 small populations known, highly susceptible to dieback
B. chamaephyton was previously declared rare, but was removed from the list after the discovery of further populations; it continues to be monitored.
Taxonomy
The Banksia genus was first described and named by Carolus Linnaeus the Younger in his April 1782 publication Supplementum Plantarum systematis vegetabilium; hence the full name for the genus is "Banksia L.f." The genus name honours the English botanist Sir Joseph Banks, who collected the first Banksia specimens in 1770, during James Cook's first expedition. The name Banksia had in fact already been published in 1775 as Banksia J.R.Forst. & G.Forst, referring to the genus now known as Pimelia. Despite this prior claim, by 1800 Linnaeus' use was generally accepted. The name was challenged in 1891 by Kuntze, who proposed the name Sirmuellera, but without success. In 1940, Banksia L.f. was formally conserved against Banksia J.R.Forst. & G.Forst by T. A. Sprague.
The exact number of species of Banksia is a matter of some contention. The most recent authoritative synopsis, George (1999), listed 76 species. Since then a new species, B. rosserae, has been published (Olde and Marriott, 2002), bringing the number of species to 77. However George (1999) gave subspecific rank to four taxa that had previously been promoted to species in Thiele and Ladiges (1996). Most Australian herbaria have continued to follow Thiele and Ladiges (1996) in ranking these taxa as species, thus recognising 81 species. Furthermore Harden (2002) recognises the species B. cunninghamii Sieber ex Reichb, even though both George (1999) and Theile and Ladiges (1996) consider it a subspecies of B. spinulosa. Finally, B. paludosa subsp. astrolux is under review and is likely to be raised to species level soon.
The Banksia genus is divided into two subgenera: Banksia subg. Isostylis and Banksia subg. Banksia. The former subgenus contains three closely related species, all from Western Australia, that have head-like flower clusters instead of Banksia's characteristic flower spikes. The flower heads of Banksia subg. Isostylis are superficially similar to those of Dryandra species, and it is generally accepted that the subgenus provides an evolutionary link to Dryandra. The other subgenus, Banksia subg. Banksia, contains the remaining species, all of which have the characteristic Banksia flower spikes. These are split into three sections, primarily on the shape of the style:
- Banksia sect. Banksia species have a straight or curved, but never hooked, style. This section contains about 50 species that are further divided into nine series;
- Banksia sect. Coccinea contains a single species, Banksia coccinea;
- Banksia sect. Oncostylis contains those species whose styles are hooked. It contains about 20 species, which are further divided into four series.
For a full taxonomic arrangement of Banksia according to George(1999), see Taxonomy of Banksia. For an alphabetically ordered list of Banksia species, with common names and including species not recognised by George (1999), see List of Banksia species.
Uses and cultural references
Banksias are popular garden plants in Australia because of their large, showy flower heads, and because the large amounts of nectar they produce attracts birds and small mammals. Popular garden species include the Wallum Banksia, B. aemula, the Saw Banksia, B. serrata, and the cultivar Banksia 'Giant Candles'. Banksia species are easily propagated by seed, and many can be propagated by cutting. They require little maintenance. If fertilised, only slow-release, low-phosphorus fertilizer should be used, as the proteoid roots may be damaged by high nutrient levels in the soil.
With the exception of the nursery industry, Banksia have limited commercial use. Some species are grown on wildflower farms, and the flower heads harvested for the cut flower trade. Their nectar is also sought by beekeepers, not for the quality of the honey, which is often poor, but because the trees provide an abundant and reliable source of nectar at times when other sources provide little.
Banksia wood is a red colour with an attractive grain, but it is rarely used as it warps badly on drying. It is occasionally used for ornamental purposes, such as turnery and cabinet panelling, and has also been used to make keels for small boats. The "cones" are often sliced up and sold as coasters; these are generally marketed as souvenirs for international tourists.
The Aboriginal people of south-western Australia would suck on the flower spikes to obtain the nectar. They also ate grubs extracted from the trees.
Perhaps the best known cultural reference to Banksia is the "Banksia men" of May Gibbs' children's book Snugglepot and Cuddlepie. Gibb's Banksia men are modelled on the appearance of aged Banksia "cones", with follicles for eyes and other facial features.
Gallery
 Banksia flower spikes. Release of the flower styles has commenced at the top of the flower spike on the right. |
 Banksia head |
 Dried up banksia |
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External links
References
- {{{Author|}}}{{|{{{3}}}}}}|show1| (1984)}}{{{{{Year|}}}}}}|show1|.}} {{|{{{3}}}}}}|show1|[{{{URL}}}}} Forest Trees of Australia (Fourth edition revised and enlarged){{|{{{3}}}}}}|show1|]}}{{|{{{3}}}}}}|show1|, {{{Pages}}}}}{{|{{{3}}}}}}|Show1|, CSIRO Publishing, Collingwood, Victoria, Australia}}. {{{ID|}}}.
- George, A. S. (1981). The Genus Banksia. Nuytsia 3 (3): 239–473.
- George, A. S. (1999). "Banksia". In "Wilson, Annette (ed.), Flora of Australia: Volume 17B: Proteaceae 3: Hakea to Dryandra, 175–251. CSIRO Publishing / Australian Biological Resources Study. ISBN 0643064540.
- Harden, Gwen (2002). "Banksia". In "Harden, Gwen (ed), Flora of New South Wales: Volume 2 (Revised Edition), 82–86. New South Wales University Press, Kensington. ISBN 0868401560.
- Olde, Peter M. and Marriott, Neil R. (2002). One new Banksia and two new Grevillea species (Proteaceae: Grevilleoideae) from Western Australia. Nuytsia 15 (1): 85–99.
- {{{Author|}}}{{|{{{3}}}}}}|show1| (1988)}}{{{{{Year|}}}}}}|show1|.}} {{|{{{3}}}}}}|show1|[{{{URL}}}}} The Banksia Atlas (Australian Flora and Fauna Series Number 8){{|{{{3}}}}}}|show1|]}}{{|{{{3}}}}}}|show1|, {{{Pages}}}}}{{|{{{3}}}}}}|Show1|, Australian Government Publishing Service, Canberra}}. {{{ID|}}}
- Thield, Kevin and Ladiges, Pauline Y. (1996). A Cladistic Analysis of Banksia (Proteaceae). Australian Systematic Botany 9: 661–733.
- {{{Author|}}}{{|{{{3}}}}}}|show1| (1990)}}{{{{{Year|}}}}}}|show1|.}} {{|{{{3}}}}}}|show1|[{{{URL}}}}} Leaf and Branch: Trees and Tall Shrubs of Perth{{|{{{3}}}}}}|show1|]}}{{|{{{3}}}}}}|show1|, {{{Pages}}}}}{{|{{{3}}}}}}|Show1|, Department of Conservation and Land Management, Perth, Western Australia}}. {{{ID|}}}
