Xerochrysum Bracteatum - Description

Description

The plant is an erect perennial, or occasionally annual, herb that is simple or rarely branched at its base. It generally reaches 20 to 80 cm (8–32 in) in height, but can have a prostrate habit in exposed areas such as coastal cliffs. The green stems are rough and covered with fine hairs, and are robust compared with those of other members of the genus. The leaves are lanceolate, elliptic or oblanceolate in shape and measure anywhere from 1.5 to 10 cm (0.6–4 in) long and from 0.5 to 2 cm (0.2–0.8 in) wide. They are also covered with cobwebby hairs. Sitting atop tall stems above the foliage, the flower heads range from 3 to 7 cm (1.2–2.8 in) in diameter. Occasionally multiple heads arise from the one stem. Like the flowers of all Asteraceae, they are composed of a central disc which contains a number of tiny individual flowers, known as florets; these sit directly on an enlarged part of the stem known as the receptacle.

Around the disc is an involucre of modified leaves, the bracts, which in Xerochrysum, as in most Gnaphalieae, are petal-like, stiff and papery. Arranged in rows, these bracts curl over and enclose the florets, shielding them before flowering. They create the impression of a shiny and yellow corolla around the disc. The intermediate bracts are sometimes white, while the outer ones are paler and often streaked reddish or brown (a greater variety of colours are found in cultivars). These bracts are papery and dry, or scarious, with a low water content, unlike leaves or flower parts of other plants. They are made up of dead cells, which are unusual in that they have a thin primary and a thick secondary cell wall, a feature only found in sclerenchyma, or structural, cells, not cells of flowers or leaves.

The individual florets are yellow. Those on the outer regions of the disc are female, while those in the centre are bisexual. Female flowers lack stamens and have only a very short tube-shaped corolla surrounding a pistil that splits to form two stigmas, while bisexual or hermaphrodite flowers have a longer corolla, and (as in virtually all members of the family) five stamens fused at the anthers, with the pistil emerging from the center. The yellow corolla and pistil are located above an ovary with a single ovule, and surrounded by the pappus, the highly modified calyx of Asteraceae. It comprises a number of bristles radiating around the florets. Yellow in colour, they persist and are thought to aid in the wind dispersal of the 0.3 cm (0.1 in) long fruit. The smooth brown fruit, known as a cypsela, is 2 to 3 mm long with the pappus radiating from one end.

In the wild, X. bracteatum can be distinguished from X. bicolor in Tasmania by its broader leaves and cobwebby hairs on the stems, and from X. macranthum in Western Australia by the flower head colour; the latter species has white flower heads whereas those of X. bracteatum are golden yellow. Xerochrysum subundulatum from alpine and subalpine areas of New South Wales, Victoria and Tasmania is rhizomatous, and has markedly pointed orange bracts. The eastern Australian species Xerochrysum viscosum can be distinguished by its rough and sticky leaves.