Banksia
Banksia is a genus of around 170 species of flowering plants in the family Proteaceae. These Australian wildflowers and popular garden plants are easily recognised by their characteristic flower spikes, and woody fruiting "cones" and heads. Banksias range in size from prostrate woody shrubs to trees up to 30 metres tall. They are found in a wide variety of landscapes: sclerophyll forest, rainforest, shrubland, and some more arid landscapes, though not in Australia's deserts.
Heavy producers of nectar, banksias are a vital part of the food chain in the Australian bush. They are an important food source for nectarivorous animals, including birds, bats, rats, possums, stingless bees and a host of invertebrates. Further, they are of economic importance to Australia's nursery and cut flower industries. However, these plants are threatened by a number of processes including land clearing, frequent burning and disease, and a number of species are rare and endangered.
Description
Banksias grow as trees or woody shrubs. Trees of the largest species, B. integrifolia and B. seminuda, often grow over 15 metres tall, some even grow to standing 30 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– centimetre long needle-like leaves of B. ericifolia, to the very large leaves of 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. Many species have differing juvenile and adult leaves.
The flowers are arranged in flower spikes or capitate flower heads. The character most commonly associated with Banksia is the flower spike, an elongated inflorescence consisting of a woody axis covered in tightly packed pairs of flowers attached at right angles. A single flower spike generally contains hundreds or even thousands of flowers; the most recorded is around 6000 on inflorescences of B. grandis. Not all Banksia have an elongate flower spike, however: the members of the small Isostylis complex have long been recognised as banksias in which the flower spike has been reduced to a head; and recently the large genus Dryandra has been found to have arisen from within the ranks of Banksia, and sunk into it as B. ser. Dryandra. They similarly have capitate flower heads rather than spikes.
File:Banksia marginata immature and mature.jpg|thumb|B. marginata flower spike before and after anthesis
Banksia flowers are usually a shade of yellow, but orange, red, pink and even violet 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. This can be most spectacular in B. prionotes and related species, as the white inflorescence in bud becomes a brilliant orange. In most cases, the individual flowers are tall, thin saccate in shape.
File:Banksia Marginata Seed.jpg|thumb|left|Seed separator of a silver banksia with winged seeds still cohering
Occasionally, multiple flower spikes can form. This is most often seen in Banksia marginata and B. ericifolia ''''.
File:Banksia integrifolia and marginata cones.jpg|thumb|Infructescence of B. integrifolia, with non-persistent flowers; and B. marginata, with persistent flowers
As the flower spikes or heads age, the flower parts dry up and may turn shades of orange, tan or dark brown colour, before fading to grey over a period of years. In some species, old flower parts are lost, revealing the axis; in others, the old flower parts may persist for many years, giving the fruiting structure a hairy appearance. Old flower spikes are commonly referred to as "cones", although they are not technically cones according to the botanical definition of the term: cones only occur in conifers and cycads.
Despite the large number of flowers per inflorescence, only a few of them ever develop fruit, and in some species a flower spike will set no fruit at all. The fruit of Banksia is a woody follicle embedded in the axis of the inflorescence. In many species, the resulting structure is a massive woody structure commonly called a cone. Each follicle consists of two horizontal valves that tightly enclose the seeds. The follicle opens to release the seed by splitting along the suture. 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 one or two small seeds, each with a wedge-shaped papery wing that causes it to spin as it falls to the ground.
Taxonomy
Specimens of Banksia were first collected by Sir Joseph Banks and Daniel Solander, naturalists on the Endeavour during Lieutenant James Cook's first voyage to the Pacific Ocean. Cook landed on Australian soil for the first time on 29 April 1770, at a place that he later named Botany Bay in recognition of "the great quantity of plants Mr Banks and Dr Solander found in this place". Over the next seven weeks, Banks and Solander collected thousands of plant specimens, including the first specimens of a new genus that would later be named Banksia in Banks' honour. Four species were present in this first collection: B. serrata, B. integrifolia, B. ericifolia and B. robur. In June the ship was careened at Endeavour River, where specimens of B. dentata were collected.The genus Banksia was finally described and named by Carolus Linnaeus the Younger in his April 1782 publication Supplementum Plantarum; hence the full name for the genus is "Banksia L.f.". Linnaeus placed the genus in class Tetrandra, order Monogynia of his father's classification, and named it in honour of Banks. The name Banksia had in fact already been published in 1775 as Banksia J.R.Forst & G.Forst, referring to some New Zealand species that the Forsters had collected during Cook's second voyage. However Linnaeus incorrectly attributed the Forsters' specimens to the genus Passerina, and therefore considered the name Banksia available for use. By the time Joseph Gaertner corrected Banks' error in 1788, Banksia L.f. was widely known and accepted, so Gaertner renamed Banksia J.R.Forst, & G.Forst to Pimelea, a name previously chosen for the genus by Banks and Solander.
The first specimens of a Dryandra were collected by Archibald Menzies, surgeon and naturalist to the Vancouver Expedition. At the request of Joseph Banks, Menzies collected natural history specimens wherever possible during the voyage. During September and October 1791, while the expedition were anchored at King George Sound, he collected numerous plant specimens, including the first specimens of Dryandra sessilis and D. pellaeifolia. Upon Menzies' return to England, he turned his specimens over to Banks; as with most other specimens in Banks' library, they remained undescribed for many years. Robert Brown gave a lecture, naming the new genus Dryandra in 1809, however Joseph Knight published the name Josephia before Brown published his paper with the description of Dryandra. Brown ignored Knight's name, as did subsequent botanists.
In 1891, Otto Kuntze, strictly applying the principle of priority, argued that Pimelea should revert to the name Banksia J.R.Forst & G.Forst. He proposed the new genus Sirmuellera to replaced Banksia L.f. and transferred its species to the new genus. This arrangement was largely ignored by Kuntze's contemporaries.Banksia L.f. was formally conserved and Sirmuellera rejected in 1940.
Banksia belongs to the family Proteaceae, subfamily Grevilleoideae, and tribe Banksieae. There are around 170 species. The closest relatives of Banksia are two genera of rainforest trees in North Queensland.
Subgeneric arrangement
arranged the genus into two subgenera—subgenus Isostylis and subgenus Banksia —in his 1981 monograph and 1999 treatment for the Flora of Australia series. He held that flower morphology was the key to relationships in the genus. Austin Mast and Kevin Thiele published the official merging of Dryandra within Banksia in 2007, recalibrating the genus into subgenus Banksia and subgenus Spathulatae.Distribution and habitat
All but one of the living Banksia species are endemic to Australia. The exception is B. dentata, which occurs throughout northern Australia, and on islands to the north including New Guinea and the Aru Islands. An extinct species, B. novae-zelandiae, was found in New Zealand. 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 90% of all Banksia species occur only there, from Exmouth in the north, south and east to beyond Esperance on the south coast. Eastern Australia has far fewer species, but these include some of best known and most widely distributed species, including B. integrifolia and B. spinulosa. Here they occur from the Eyre Peninsula in South Australia right around the east coast up to Cape York in Queensland.The vast majority of Banksia are found in sandy or gravelly soils, though some populations of B. marginata and B. spinulosa do occur on soil that is heavier and more clay-like. B. seminuda is exceptional for its preference for rich loams along watercourses.
Most occur in heathlands or low woodlands; of the eastern species, B. integrifolia and B. marginata occur in forests; many south-western species such as B. grandis, B. sphaerocarpa, B. sessilis, B. nobilis and B. dallanneyi grow as understorey plants in jarrah, wandoo and karri forests, with B. seminuda being one of the forest trees in suitable habitat.
Most species do not grow well near the coast, notable exceptions being the southern Western Australian species B. speciosa, B. praemorsa and B. repens. Only a few species, such as B. rosserae and B. elderiana, occur in arid areas. Most of the eastern Australian species survive in uplands, but only a few of the Western Australian species native to the Stirling Ranges – B. solandri, B. oreophila, B. brownii and B. montana'' ''– survive at high elevations.
Studies of the south-western species have found the distribution of Banksia species to be primarily constrained by rainfall. With the exception of B. rosserae, no species tolerates annual rainfall of less than 200 millimetres, despite many species surviving in areas that receive less than 400 millimetres. Banksia species are present throughout the region of suitable rainfall, with greatest speciation in cooler, wetter areas. Hotter, drier regions around the edges of its range tend to have fewer species with larger distributions. The greatest species richness occurs in association with uplands, especially the Stirling Range.