Banksia marginata

Silver banksia
Banksia marginata immature and mature.jpg
Inflorescence with unopened buds (left), opened flowers (right)
Scientific classification edit
Kingdom:Plantae
Clade:Tracheophytes
Clade:Angiosperms
Clade:Eudicots
Order:Proteales
Family:Proteaceae
Subfamily:Grevilleoideae
Tribe:Banksieae
Genus:Banksia
Species:
B. marginata
Binomial name
Banksia marginata
Synonyms[1]

Banksia microstachya Cav.
Banksia depressa R.Br.
Banksia insularis R.Br.
Banksia patula R.Br.
Banksia gunnii Meisn.

Banksia marginata,[1] commonly known as the silver banksia, is a species of tree or woody shrub in the plant genus Banksia found throughout much of southeastern Australia. It ranges from the Eyre Peninsula in South Australia to north of Armidale, New South Wales, and across Tasmania and the islands of Bass Strait. It grows in various habitats, including Eucalyptus forest, scrub, heathland and moorland. Banksia marginata varies widely in habit, ranging from a 20-centimetre (7.9 in) shrub to a 12-metre (40 ft) tree. The narrow leaves are linear and the yellow inflorescences (flower spikes) occur from late summer to early winter. The flower spikes fade to brown and then grey and develop woody follicles bearing the winged seeds. Originally described by Antonio José Cavanilles in 1800, further collections of B. marginata were designated as several separate species by Robert Brown in 1810. However, all were reclassified as a single species by George Bentham in 1870. No distinct subspecies have been recognised by Banksia expert Alex George, who nonetheless concedes that further work is needed.

Many species of bird, in particular honeyeaters, forage at the flower spikes, as do native and European honeybees. The response to bushfire varies. Some populations are serotinous: they are killed by fire and regenerate from large stores of seed which have been held in cones in the plant canopy and are released after a fire. Others regenerate from underground lignotubers or suckers from lateral roots. Although it has been used for timber, Banksia marginata is most commonly seen as a garden plant, with dwarf forms being commercially propagated and sold.

Description

Tree in grassland
Tree habit, Illabarook Rail Line Nature Conservation Reserve

Banksia marginata is a highly variable species, usually ranging from a small shrub around a metre (3 ft) tall to a 12-metre-high (39 ft) tree.[2] Unusually large trees of 15 to possibly 30 m (50–100 ft) have been reported near Beeac in Victoria's Western District as well as several locations in Tasmania,[3] while compact shrubs limited to 20 cm (7.9 in) high have been recorded on coastal heathland in Tasmania (such as at Rocky Cape National Park).[4] Shrubs reach only 2 m (6.6 ft) high in Gibraltar Range National Park.[5] The bark is pale grey and initially smooth before becoming finely tessellated with age. The new branchlets are hairy at first but lose their hairs as they mature,[2] with new growth a pale or pinkish brown.[6] The leaves are alternately arranged on the stems on 2–5 mm long petioles, and characteristically toothed in juvenile or younger leaves (3–7 cm [1.2–2.8 in] long). The narrow adult leaves are dull green in colour and generally linear, oblong or wedge-shaped (cuneate) and measure 1.5–6 cm (0.6–2.4 in) long and 0.3–1.3 cm (0.1–0.5 in) wide. The margins become entire with age, and the tip is most commonly truncate or emarginate, but can be acute or mucronate.[7] The cellular makeup of the leaves shows evidence of lignification, and the leaves themselves are somewhat stiff.[8] Leaves also have sunken stomates. The leaf undersurface is white with a prominent midrib covered in brownish hairs.[7]

The complex flower spikes, known as inflorescences, appear generally from late summer to early winter (February to June) in New South Wales and Victoria, although flowering occurs in late autumn and winter in the Gibraltar Range.[5] Cylindrical in shape, they are composed of a central woody spike or axis, perpendicularly from which a large number of compact floral units arise, which measure 5–10 cm (2–4 in) tall and 4–6 cm (1.6–2.4 in) wide.[7] Pale yellow in colour, they are composed of up to 1,000 individual flowers (784 recorded in the Gibraltar Range[5]) and arise from nodes on branchlets that are at least three years old. Sometimes two may grow from successive nodes in the same flowering season. They can have a grey or golden tinge in late bud. As with most banksias, anthesis is acropetal; the opening of the individual buds proceeds up the flower spike from the base to the top.[7] Over time the flower spikes fade to brown and then grey, and the old flowers generally persist on the cone.[9] The woody follicles grow in the six months after flowering, with up to 150 developing on a single flower spike. In many populations, only a few follicles develop. Small and elliptic, they measure 0.7–1.7 cm (0.3–0.7 in) long, 0.2–0.5 cm (0.1–0.2 in) high, and 0.2–0.4 cm (0.1–0.2 in) wide.[7] In coastal and floodplain populations, these usually open spontaneously and release seed, while they generally remain sealed until burnt by fire in plants from heathland and montane habitats.[10] Each follicle contains one or two fertile seeds, between which lies a woody dark brown separator of similar shape to the seeds. Measuring 0.9–1.5 cm (0.4–0.6 in) in length, the seed is egg- to wedge-shaped (obovate to cuneate) and composed of a dark brown 0.8–1.1 cm (0.3–0.4 in) wide membranous "wing" and wedge- or sickle-shaped (cuneate–falcate) seed proper which measures 0.5–0.8 cm (0.2–0.3 in) long by 0.3–0.4 cm (0.1–0.2 in) wide. The seed surface can be smooth or covered in tiny ridges, and often glistens. The resulting seedling first grows two obovate cotyledon leaves, which may remain for several months as several more leaves appear.[7] The cotyledons of Banksia marginata, B. paludosa and B. integrifolia are very similar in appearance.[11]

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