Banksia prionotes, commonly known as acorn banksia or orange banksia, is a species of shrub or tree of the genus Banksia in the family Proteaceae. It is native to the southwest of Western Australia and can reach up to in height. It can be much smaller in more exposed areas or in the north of its range. This species has serrated, dull green leaves and large, bright flower spikes, initially white before opening to a bright orange. Its common name arises from the partly opened inflorescence, which is shaped like an acorn. The tree is a popular garden plant and also of importance to the cut flower industry.
Banksia prionotes was first described in 1840 by English botanist John Lindley, probably from material collected by James Drummond the previous year. There are no recognised varieties, although it has been known to hybridise with Banksia hookeriana. Widely distributed in south-west Western Australia, B. prionotes is found from Shark Bay (25° S) in the north, south as far as Kojonup (33°50′S). It grows exclusively in sandy soils, and is usually the dominant plant in scrubland or low woodland. Pollinated by birds, it provides food for a wide array of vertebrate and invertebrate animals in the autumn and winter months. It is an important source of food for honeyeaters (Meliphagidae), and is critical to their survival in the Avon Wheatbelt region, where it is the only nectar-producing plant in flower at some times of the year.
Description Banksia prionotes grows as a tree up to high in southern parts of its distribution, but in northern parts it is usually a shorter tree or spreading shrub, reaching about in height; it diminishes in size as the climate becomes warmer and drier further north. It has thin, mottled grey, smooth or grooved bark, and tomentose young stems. The alternate dull green leaves are long, and wide, with toothed leaf margins made up of triangular lobes, and often a wavy surface.
Flowers occur in a typical Banksia flower spike, an inflorescence made up of hundreds of small individual flowers, or florets, densely packed around a cylindrical axis. B. prionotes has cream-coloured flowers with a bright orange limb that is not revealed until the flower fully opens. Known as anthesis, this process sweeps through the inflorescence from bottom to top over a period of days, creating the effect of a cream inflorescence that progressively turns bright orange. The old flower parts fall away after flowering finishes, revealing the axis, which may bear up to 60 embedded follicles. Oval or oblong in shape and initially covered in fine hairs, these follicles are from long and wide, and protrude from the cone. Inside, they bear two seeds separated by a brownish woody seed separator. The matte blackish seeds are wedge-shaped (cuneate) and measure long by wide with a membranous 'wing'.
The root system consists of a main sinker root, and up to ten lateral roots extending from a non-lignotuberous root crown. The main sinker root grows straight down to the water table; it may be up to long if the water table is that deep. Typically from in diameter immediately below the root crown, roots become gradually finer with depth, and may be less than half a centimetre (0.2 in) wide just above the water table. Upon reaching the water table, the sinker branches out into a network of very fine roots. The laterals radiate out horizontally from the base of the plant, at a depth of . They may extend over from the plant, and may bear secondary laterals; larger laterals often bear auxiliary sinker roots. Lateral roots seasonally form secondary rootlets from which grow dense surface mats of proteoid roots, which function throughout the wetter months before dying off with the onset of summer.
Taxonomy
Banksia prionotes was first published by English botanist John Lindley in the January 1840 issue of his A Sketch of the Vegetation of the Swan River Colony; hence the species' standard author citation is Banksia prionotes Lindl. He did not specify the type material upon which he based the species, but A Sketch of the Vegetation of the Swan River Colony is based primarily upon the collections of early settler and botanist James Drummond. A sheet of mounted specimens at the University of Cambridge Herbarium (CGE), labelled "Swan River, Drummond, 1839" and annotated "Banksia prionotes m" in Lindley's hand, has since been designated the lectotype. Lindley also made no mention of the etymology of the specific epithet prionotes, but it is assumed to be derived from the Ancient Greek ('saw') and ('quality'), referring to the serrated leaf margins.
The most frequently reported common names of B. prionotes are acorn banksia, derived from the resemblance of partly opened inflorescences to acorns; and orange banksia. Other reported common names include saw-toothed banksia and golden banksia Bwongka is a generic Noongar name for Banksia in the Avon River catchment, where B. prionotes is one of several species occurring.
No further subspecies or varieties of B. prionotes have been described, and it has no taxonomic synonyms. Its only nomenclatural synonym is Sirmuellera prionotes (Lindl.) Kuntze, which arose from Otto Kuntze's unsuccessful 1891 attempt to transfer Banksia into the new name Sirmuellera. When Carl Meissner published his infrageneric arrangement of Banksia in 1856, he placed B. prionotes in section Eubanksia because its inflorescence is a spike rather than a domed head, and in series Salicinae, a large series that is now considered quite heterogeneous. This series was discarded in the 1870 arrangement of George Bentham; instead, B. prionotes was placed in section Orthostylis, which Bentham defined as consisting of those Banksia species with flat leaves with serrated margins, and rigid, erect styles that "give the cones after the flowers have opened a different aspect". In 1981, Alex George published a revised arrangement that placed B. prionotes in the subgenus Banksia because of its flower spike, section Banksia because its styles are straight rather than hooked, and the series Crocinae, a new series of four closely related species, all with bright orange perianths and pistils.
George's arrangement remained current until 1996, when Kevin Thiele and Pauline Ladiges published an arrangement informed by a cladistic analysis of morphological characteristics. Their arrangement maintained B. prionotes in B. subg. Banksia, but discarded George's sections and his series Crocinae. Instead, B. prionotes was placed at the end of series Banksia, in subseries Cratistylis. Questioning the emphasis on cladistics in Thiele and Ladiges' arrangement, George published a slightly modified version of his 1981 arrangement in his 1999 treatment of Banksia for the Flora of Australia series of monographs. To date, this remains the most recent comprehensive arrangement. The placement of B. prionotes in George's 1999 arrangement may be summarised as follows:
Banksia B. subg. Banksia B. sect. Banksia B. ser. Salicinae (11 species, 7 subspecies) B. ser. Grandes (2 species) B. ser. Banksia (8 species) B. ser. Crocinae B. prionotes B. burdettii B. hookeriana B. victoriae B. ser. Prostratae (6 species, 3 varieties) B. ser. Cyrtostylis (13 species) B. ser. Tetragonae (3 species) B. ser. Bauerinae (1 species) B. ser. Quercinae (2 species) B. sect. Coccinea (1 species) B. sect. Oncostylis (4 series, 22 species, 4 subspecies, 11 varieties) B. subg. Isostylis (3 species)
Since 1998, American botanist Austin Mast has been publishing results of ongoing cladistic analyses of DNA sequence data for the subtribe Banksiinae, which includes Banksia. With respect to B. prionotes, Mast's results are fairly consistent with those of both George and Thiele and Ladiges. Series Crocinae appears to be monophyletic, and B. hookeriana is confirmed as B. prionotes closest relative. Overall, however, the inferred phylogeny is very different from George's arrangement. Early in 2007, Mast and Thiele initiated a rearrangement of Banksiinae by publishing several new names, including subgenus Spathulatae for the species of Banksia that have spoon-shaped cotyledons; in this way they also redefined the autonym B. subgenus Banksia. They have not yet published a full arrangement, but if their nomenclatural changes are taken as an interim arrangement, then B. prionotes is placed in subgenus Banksia.
Hybrids
With Banksia hookeriana Banksia prionotes readily hybridises with Banksia hookeriana (Hooker's banksia) under experimental conditions, indicating that these species have highly compatible pollen. The cultivar B. 'Waite Orange' is believed to be such a hybrid, having arisen by open pollination during a breeding experiment conducted at the Waite Agricultural Research Institute of the University of Adelaide in 1988.
Banksia prionotes × hookeriana has also been verified as occurring in the wild, but only in disturbed locations. The two parent species have overlapping ranges and are pollinated by the same honeyeater species; and though preferring different soils, they often occur near enough to each other for pollinators to move between them. It therefore appears that the only barrier to hybridisation in undisturbed areas is the different flowering seasons: B. prionotes has usually finished flowering by the end of May, whereas flowering of B. hookeriana usually does not begin until June. In disturbed areas, however, the increased runoff and reduced competition mean extra nutrients are available, and this results in larger plants with more flowers and a longer flowering season. Thus the flowering seasons overlap, and the sole barrier to interbreeding is removed. The resultant F1 hybrids are fully fertile, with seed production rates similar to that of the parent species. There is no barrier to backcrossing of hybrids with parent species, and in some populations this has resulted in hybrid swarms. This raises the possibility of the parent species gradually losing their genetic integrity, especially if the intermediate characteristics of the hybrid offer it a competitive advantage over the parent species, such as a wider habitat tolerance. Moreover, speciation might occur if the hybrid's intermediate characteristics allow it to occupy a habitat unsuited to both parents, such as an intermediate soil type.
Banksia prionotes × hookeriana hybrids have characteristics intermediate between the two parents. For example, the first putative hybrids studied had a habit "like that of gigantic B. hookerana [sic]", having inherited the size of B. prionotes, together with B. hookerianas tendency to branch from near the base of the trunk. Similarly, the infructescences were like B. prionotes in size, but had persistent flowers like B. hookeriana. Inflorescences and leaves were intermediate in size and shape, and bark was like that of B. prionotes.
Other putative hybrids
During data collection for The Banksia Atlas project, a single presumed natural hybrid between B. prionotes and B. lindleyana (porcupine banksia), with fruit like B. lindleyana but leaves intermediate between the two parents, was found north of Kalbarri National Park. At the time this was considered an important discovery, as the parent species were thought not to be closely related. Mast's analyses, however, place them both in a clade of eight species, though B. lindleyana remains less closely related to B. prionotes than B. hookeriana. Hybrids of B. prionotes with B. menziesii (firewood banksia) have also been produced by artificial means, and presumed natural hybrids have been recorded.
Distribution and habitat