Banksia subg. Isostylis is a subgenus of Banksia. It contains three closely related species, all of which occur only in SouthwestWestern Australia. Members of subgenus Isostylis have dome-shaped flower heads that are superficially similar to those of B.ser. Dryandra, but structurally more like reduced versions of the "flower spikes" characteristic of most other Banksia taxa.
There are three species of Banksia subg. Isostylis, B.ilicifolia (holly-leaved banksia), B.cuneata (matchstick banksia) and B.oligantha (Wagin banksia). B.ilicifolia is widely distributed and relatively common, but the other two species are rare and threatened.
Description
Banksiasubg. Isostylis shares with B.ser. Dryandra the property of having compact, dome-shaped flower heads. Structurally, however, Isostylis flower heads are quite different from those of B.ser. Dryandra, having more in common with the erect flower spikes of other Banksia taxa. Specifically, Isostylis flower heads have an ovoid axis, suggestive of a greatly reduced flower spike, whereas Dryandra flower heads emerge from a flat receptable. Furthermore, Isostylis has thick follicles with a woolly coating, whereas Dryandra follicles are thin and hairless; and the involucral, common and floral bracts of Isostylis are unlike those of Dryandra.[1]
The Isostylis species are all upright shrubs or trees, with a single trunk. They generally have serrate leaves, although in rare cases B.ilicifolia may have entire leaves.[2]
Banksiasubg. Isostylis was first published by Robert Brown in his 1810 Prodromus Florae Novae Hollandiae et Insulae Van Diemen; thus its full name, with author citation, is Banksia subg. Isostylis R.Br.[3]Brown's arrangement was the first infrageneric arrangement of Banksia, making B.subg. IsostylisBanksia's first infrageneric taxon.[4] Brown erected B.subg. Isostylis to contain B.ilicifolia, which was then the only known Banksia with a dome-shaped inflorescence. He did not explicitly name a type species for the subgenus,[4] but B.ilicifolia is treated as the type because it was the only member when the subgenus was published.[1]
Twenty years later, Brown issued a supplement to his Prodromus entitled Supplementum Primum Prodromi Florae Novae Hollandiae; another nine Banksia species were published, but there was no change to the 1810 arrangement, and no new Isostylis species.[5]
In 1905, James Britten challenged the genus name Banksia, on the grounds that BanksiaJ.R.Forst & G.Forst had precedent over BanksiaL.f. Britten adopted the name Isostylis for the entire genus, republishing the names Isostylis dentata (L.f.) Britten for B.dentata (Tropical Banksia), Isostylis ericifolia (L.f.) Britten for B.ericifolia (Heath-leaved Banksia), Isostylis integrifolia (L.f.) Britten for B.integrifolia (Coast Banksia) and Isostylis serrata (L.f.) Britten for B.serrata (Saw Banksia). This challenge failed, Banksia L.f. was eventually conserved, and his four names are now considered taxonomic synonyms of their respective names under Banksia L.f. None of them are considered members of Isostylis.
The next change to Isostylis came in 1981, when Alex George promoted it back to subgenus rank, and published a second species, B.cuneata. In discussing the subgenus, George commented that there had been calls to transfer Isostylis into Dryandra, which was then a distinct genus. He argued, however, that the similarities between Isostylis and Dryandra were largely superficial, whereas the similarities with Banksia were much more important taxonomically. His conclusion was that the taxon should remain in Banksia, although he did not rule out promoting it into a separate genus. A third B.subg. Isostylis species, B.oligantha, was published by George in 1988.
In 1996, Kevin Thiele and Pauline Ladiges published a revised arrangement based on a cladistic analysis of morphological characters of Banksia. They took up the question of an affinity of Isostylis with Dryandra, finding George's arguments unconvincing but failing to find any further evidence for or against Isostylis's placement within Banksia. They eventually accepted both of George's subgenera, using each as an outgroup in the analysis of the other. Thus their analysis yielded no information about the circumscription and placement of Isostylis, and their arrangement maintained Isostylis as a subgenus.[7]
Thiele and Ladiges' arrangement was not accepted by George, and was largely discarded by him in his 1999 arrangement. The placement and circumscription of B.subg. Isostylis was unaffected and can be summarised as follows:[2]
B.cuneata grows as a large shrub, up to three metres in height.
Since 1998, Austin Mast has been publishing results of ongoing cladistic analyses of DNA sequence data for the subtribe Banksiinae. His analyses has provided compelling evidence for the paraphyly of Banksia with respect to Dryandra, and suggest an overall phylogeny that is very greatly different from George's arrangement. Mast's results clearly recognise Isostylis as a distinct clade, but place it in a fairly recent position, within a clade that also contains B.elegans (Elegant Banksia) and B.attenuata (Candlestick Banksia). It falls a substantial distance from Dryandra, suggesting that similarities between those two groups are indeed superficial.[8][9][10]
Early in 2007, Mast and Thiele initiated a rearrangement of Banksia by merging Dryandra into it, and publishing B.subg. Spathulatae for the species having spoon-shaped cotyledons. They foreshadowed publishing a full arrangement once DNA sampling of Dryandra was complete; in the meantime, B.subg. Isostylis has been set aside. If maintained at all in Mast and Thiele's forthcoming arrangement, it will be at a lesser rank than subgenus.[11]
Relationships within B.subg. Isostylis still remain unclear. Though Mast's studies found B.cuneata to be the most basal of the three species,[9] a 2004 study of genetic divergence within the subgenus yielded both other possibilities: some analyses suggested B.ilicifolia as basal, while others suggested B.oligantha. Further complicating the situation is the existence of a population of B.cuneata having both genetic and phenetic affinities with B.oligantha. The origin of this population is unknown. It might have arisen through hybridisation, or it may be a transitional or even ancestral form. Finally, biogeographical factors suggest that B.ilicifolia would be the most basal of the three species: it occurs in the High Rainfall Zone where relictual species are most common, whereas the others are restricted to the Transitional Rainfall Zone, where more recently evolved species are most common.[12]
Distribution
Distribution of the three B.subg.Isostylis species: B.ilicifolia (red), B.cuneata (green), and B.oligantha (blue)
Ecologically, B.subg. Isostylis is similar to other Banksias. As with other Banksia taxa, all three species have proteoid roots, roots with dense clusters of short lateral rootlets that form a mat in the soil just below the leaf litter. These roots are particularly efficient at absorbing nutrients from nutrient-poor soils, such as the phosphorus-deficient native soils of Australia. They lack a lignotuber, so shrubs are killed by bushfire; mature trees of B.ilicifolia have a limited ability to resprout from epicormic buds on the trunk.[1] But like all Banksias they release their aerial seed bank following a bushfire. This adaptation, known as serotiny, ensures the rapid regeneration of populations killed by fire.
None of the B.subg. Isostylis species are popular in cultivation. The two rare species are virtually unknown in cultivation. B.ilicifolia is better known, but its usefulness as an amenity plant is limited by the fact that it has very prickly leaves.[16]
Related Research Articles
Banksia subg. Banksia is a valid botanic name for a subgenus of Banksia. As an autonym, it necessarily contains the type species of Banksia, B. serrata. Within this constraint, however, there have been various circumscriptions.
Banksia ilicifolia, commonly known as holly-leaved banksia, is a tree in the family Proteaceae. Endemic to southwest Western Australia, it belongs to Banksia subg. Isostylis, a subgenus of three closely related Banksia species with inflorescences that are dome-shaped heads rather than characteristic Banksia flower spikes. It is generally a tree up to 10 metres (33 ft) tall with a columnar or irregular habit. Both the scientific and common names arise from the similarity of its foliage to that of the English holly Ilex aquifolium; the glossy green leaves generally have very prickly serrated margins, although some plants lack toothed leaves. The inflorescences are initially yellow but become red-tinged with maturity; this acts as a signal to alert birds that the flowers have opened and nectar is available.
Banksia cuneata, commonly known as matchstick banksia or Quairading banksia, is an endangered species of flowering plant in the family Proteaceae. Endemic to southwest Western Australia, it belongs to Banksia subg. Isostylis, a sub-genus of three closely related Banksia species with inflorescences or flower clusters that are dome-shaped heads rather than characteristic Banksia flower spikes. A shrub or small tree up to 5 m (16 ft) high, it has prickly foliage and pink and cream flowers. The common name Matchstick Banksia arises from the blooms in late bud, the individual buds of which resemble matchsticks. The species is pollinated by honeyeaters (Meliphagidae).
Banksia oligantha, commonly known as Wagin banksia, is an endangered species in the plant family Proteaceae endemic to south west Western Australia. It belongs to Banksia subg. Isostylis, a subgenus of three closely related Banksia species with dome-shaped heads as inflorescences, rather than characteristic Banksia flower spikes. A shrub or small tree up to 4 m (13 ft) high, it has prickly foliage and pink and cream flowerheads which appear in late Spring.
As with other flowering plants, the taxonomy of Banksia has traditionally been based on anatomical and morphological properties of the Banksia flower, fruiting structure and seed, along with secondary characteristics such as leaf structure and growth habit. Increasingly, molecular evidence from DNA is providing important new insights into relationships within the genus and between this and other genera in the Proteaceae.
Banksia caleyi, commonly known as Caley's banksia or red lantern banksia, is a species of woody shrub of the family Proteaceae native to Western Australia. It generally grows as a dense shrub up to 2 m (7 ft) tall, has serrated leaves and red, pendent (hanging) inflorescences which are generally hidden in the foliage. First described by Scottish naturalist Robert Brown in 1830, Banksia caleyi was named in honour of the English botanist George Caley. No subspecies are recognised. It is one of three or four related species with hanging inflorescences, which is an unusual feature within the genus.
Banksia dryandroides, the dryandra-leaved banksia, is a species of small shrub in the plant genus Banksia. The Noongar peoples know the tree as manyat. It occurs in shrubland, woodland and kwongan on the south coast of Western Australia between Narrikup and Beaufort Inlet. The species is placed alone in series B. ser. Dryandroideae.
Banksia elegans, commonly known as the elegant banksia, is a species of woody shrub that is endemic to a relatively small area of Western Australia. Reaching 4 m (13 ft) high, it is a suckering shrub that rarely reproduces by seed. The round to oval yellow flower spikes appear in spring and summer. Swiss botanist Carl Meissner described Banksia elegans in 1856. It is most closely related to the three species in the subgenus Isostylis.
The taxonomy of Banksia integrifolia has a long and complex history, the result of confusion caused by the species' great variability, and similarities with some closely related species. The existence of hybrids between B. integrifolia and related species as well as early attempts to classify the species based on dried specimen material have also contributed to the confusion.
Banksia fraseri is a species of shrub that is endemic to Western Australia. It has hairy stems, broadly linear pinnatisect leaves with between four and eighteen sharply-pointed lobes on each side, between eighty and one hundred pink to cream-coloured flowers and wege-shaped follicles.
Dryandra subg. Diplophragma is an obsolete subgenus within the former genus Dryandra. It was first published by Robert Brown in 1830, but was discarded by George Bentham in 1870. It was reinstated with a new circumscription by Alex George in 1996, but was ultimately discarded again in 2007 when Austin Mast and Kevin Thiele sunk Dryandra into Banksia.
Robert Brown's taxonomic arrangement of Banksia was published in his book of 1810, Prodromus Florae Novae Hollandiae et Insulae Van Diemen, and expanded in the supplement to that publication, Supplementum Primum Prodromi Florae Novae Hollandiae, in 1830. It was the first survey of Banksia species to be published, and included descriptions of a number of previously undescribed species.
Alex George's taxonomic arrangement of Banksia was the first modern-day arrangement for that genus. First published in 1981 in the classic monograph The genus Banksia L.f. (Proteaceae), it superseded the arrangement of George Bentham, which had stood for over a hundred years. It was overturned in 1996 by Kevin Thiele and Pauline Ladiges, but restored by George in 1999. A recent publication by Austin Mast and Kevin Thiele suggests that it will soon be overturned again.
Banksia sect. Eubanksia is an obsolete section of Banksia. There have been two circumscriptions, one of which is synonymous with the recently abandoned B. subg. Banksiasensu Alex George, the other having no modern equivalent.
Robert Brown's taxonomic arrangement of Dryandra was the first arrangement of what is now Banksia ser. Dryandra. His initial arrangement was published in 1810, and a further arrangement, including an infrageneric classification, followed in 1830. Aspects of Brown's arrangements can be recognised in the later arrangements of George Bentham and Alex George.
Banksia spinulosa var. collina is a shrub that grows along the east coast of Australia, in Queensland and New South Wales. Commonly known as Hill Banksia or Golden Candlesticks, it is a taxonomic variety of B. spinulosa. It is a popular garden plant widely sold in nurseries.
Banksia spinulosa var. spinulosa is a shrub that grows along the east coast of Australia, in Queensland and New South Wales.
Banksia spinulosa var. cunninghamii, sometimes given species rank as Banksia cunninghamii, is a shrub that grows along the east coast of Australia, in Victoria and New South Wales. It is a fast-growing non-lignotuberous shrub or small tree infrequently cultivated.
Dryandra subg. Hemiclidia is an obsolete plant taxon that encompassed material that is now included in Banksia. Published at genus rank as Hemiclidia by Robert Brown in 1830, it was set aside by George Bentham in 1870, but reinstated at subgenus rank by Alex George in 1996. In 2007, all Dryandra species were transferred into Banksia at series rank, and the infrageneric Dryandra taxa, including D. subg. Hemiclidia, were set aside.
Dryandra ser. Aphragma is an obsolete series within the former genus Dryandra. It was first published at sectional rank by Robert Brown in 1830, and was retained at that rank until 1999, when Alex George demoted it to a series. It was discarded in 2007 when Austin Mast and Kevin Thiele sank Dryandra into Banksia.
↑ Mast, Austin R. (1998). "Molecular systematics of subtribe Banksiinae (Banksia and Dryandra; Proteaceae) based on cpDNA and nrDNA sequence data: implications for taxonomy and biogeography". Australian Systematic Botany. 11 (4): 321–342. doi:10.1071/SB97026.
↑ Mast, Austin R.; Eric H. Jones & Shawn P. Havery (2005). "An assessment of old and new DNA sequence evidence for the paraphyly of Banksia with respect to Dryandra (Proteaceae)". Australian Systematic Botany. CSIRO Publishing / Australian Systematic Botany Society. 18 (1): 75–88. doi:10.1071/SB04015.
↑ Broadhurst, Linda M.; Coates, David J. (2004). "Genetic divergence among and diversity within two rare Banksia species and their common close relative in the subgenus Isostylis R.Br. (Proteaceae)". Conservation Genetics. 5 (6): 837–846. doi:10.1007/s10592-004-5268-9. S2CID39559876.
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