'' is a
Subgenus of ''
Banksia ''. It contains three closely related species, all of which occur only in
Southwest Western 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.
''B.'' subg. ''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
Follicle s 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''.
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 .
1
''B.'' subg. ''Isostylis'' was first published by
Robert Brown in his 1810 ''
Prodromus Florae Novae Hollandiae Et Insulae Van Diemen ''; thus its full botanic name is .
Brown's Arrangement was the first infrageneric arrangement of ''Banksia'', making ''B.'' subg.''Isostylis'' ''Banksia''
's first infrageneric taxon.
2 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, but ''B. ilicifolia'' is treated as the type because it was the only member when the subgenus was published.
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.
3
In 1846, ''. This was not accepted, and is now considered a nomenclatural synonym of ''B. subg. ''Isostylis''.
When
Carl Meissner published
His Arrangement Of ''Banksia'' in 1856, he demoted both of Brown's subgenera to
Section al rank, maintaining as a monospecific taxon containing only ''B. ilicifolia''. Meissner's rank and circumscription of ''Isostylis'' was retained by
George Bentham in
His 1870 Arrangement for ''
Flora Australiensis '', but Bentham also published a putative variety of ''B. ilicifolia'', ''B. i.'' var. ''integrifolia'', based on specimens collected by
Ludwig Preiss near the
Swan River in
Western Australia . This was later overturned.
In 1905,
James Britten challenged the genus name ''Banksia'', on the grounds that ''Banksia''
J.R.Forst &
G.Forst had precedent over ''Banksia''
L.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 ''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.
Since 1998, '' into it, and publishing
''B.'' subg. ''Spathulatae'' for the species having spoon-shaped
Cotyledon s. 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.
7
Species of ''B.'' subg. ''Isostylis'' occur only in
Western Australia 's
South West Botanical Province . ''B. ilicifolia'' is widespread within 70 kilometres of the coast from
Mount Lesueur in the north, south to
Cape Leeuwin and east to
Albany . The other two species occur further inland, and have quite limited distributions. ''B. cuneata'' occurs
Brookton and
Bruce Rock in the
Avon Wheatbelt Biogeographic region; while ''B. oligantha'' occurs slightly further south, in the vicinity of
Wagin .
Ecologically, ''B.'' subg. ''Isostylis'' is similar to other ''Banksia''s. As with other ''Banksia'' taxa, all three species have
Proteoid Root s, 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 Bud s on the trunk. But like all ''Banksia''s they release their aerial bank of seeds following a bushfire. This adaptation, known as
Serotiny , ensures the rapid regeneration of populations killed by fire.
''B. cuneata'' and ''B. oligantha'' have been declared rare under both
Western Australia 's
Wildlife Conservation Act 1950 , and the federal
Environment Protection And Biodiversity Conservation Act 1999 . Threats include loss of habitat, ''
Phytophthora Cinnamomi '' dieback, and grazing on seedlings by
Feral Rabbits .
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.
