Description
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Banksia sessilis grows as an upright shrub or small tree up to 6 m (20 ft) high, without a lignotuber. In most varieties, new stems are covered in soft, fine hairs that are lost with maturity; but new stems of B. sessilis var. flabellifolia are usually hairless. Leaves are blue-green or dark green. Their shape differs by variety: in var. cygnorum and var. flabellifolia they are wedge-shaped, with teeth only near the apex; in var. cordata they are wedge-shaped, but with teeth along the entire margin; and in var. sessilis they are somewhat broader at the base, sometimes almost oblong in shape. Leaf size ranges from 2 to 6 cm (1–2.5 in) in length, and 0.8–4 cm (0.3–1.6 in) in width. They may be sessile (that is, growing directly from the stem without a petiole) or on a petiole up to 0.5 cm (0.2 in) long.
The inflorescences are cream or yellow, and occur in domed heads 4 to 5 cm (1½–2 in) wide, situated at the end of a stem. Each head contains from 55 to 125 individual flowers, surrounded at the base by a whorl of short involucral bracts. As with most other Proteaceae, individual flowers consist of a tubular perianth made up of four united tepals, and one long wiry style. The style end is initially trapped inside the upper perianth parts, but breaks free at anthesis. In B. sessilis the perianth is straight, 20 to 32 mm (0.8–1.3 in) long, and pale yellow. The style is slightly shorter, also straight, and cream-coloured. Thus in B. sessilis, unlike many other Banksia species, the release of the style at anthesis does not result in a showy flower colour change. One field study found that anthesis took place over four days, with the outer flowers opening first and moving inwards.
Flowering mostly takes place from July to November, although var. sessilis can start as early as May. After flowering, the flower parts wither and fall away, and up to four follicles develop in the receptable (the base of the flower head). Young follicles are covered in a fine fur, but this is lost as they mature. Mature follicles are ovoid in shape, and measure 1–1.5 cm (0.4–0.6 in) in length. Most follicles open as soon as they are ripe, revealing their contents: a woody seed separator and up to two winged seeds.
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