Xanthosia

Xanthosia pilosa, from here.

Belongs within: Araliaceae.

Southern crosses under the Southern Cross
Published 3 August 2022

Australia is well-known for its distinctive fauna—its marsupials, its monotremes, its abundance of parrots and honeyeaters. But it is also home to many distinctive plants, representing families and genera found nowhere else in the world. One such distinctly Australian genus is Xanthosia.

Species of Xanthosia are small shrubs or perennial herbs found in temperate regions of southern Australia. Species can be exceedingly variable, making their recognition difficult. Even individuals may change significantly in appearance between youth and maturity. Nevertheless, about twenty species are currently recognised. Leaves often have lobed or deeply incised margins and may be divided into three or more leaflets. Depending on the species, leaves may be either basal or cauline (that is, clustered at the base of the plant or distributed along the stalk). Species may also be either hairy or mostly smooth. Trichomes (hair cells) have a distinctive dendritic morphology (Liu et al. 2016).

The “southern cross” Xanthosia rotundifolia, copyright Tatters.

Flowers are most commonly borne in compound umbels though in some species these are reduced to simple umbels or just one to a few flowers (an umbel is the umbrella-like flowerhead that you may have seen on plants such as carrots; a compound umbel is when the central umbel stalk itself divides into a number of stalks ending in umbel-type arrangements). One species, Xanthosia rotundifolia, has been referred to by the vernacular name “southern cross” in reference to its regular arrangement of flowers in clusters of four. Flowers are white to pinkish or pale green with large, more or less ovate sepals and narrow, usually inflexed petals. Fruits are laterally compressed with paired, ribbed mericarps (Harden 1992).

Xanthosia species are found in a variety of habitats from arid to damp. Some are able to recover from fires by regrowing from their woody taproots. A handy trick in a landscape as regularly conflagrated as much of Australia.

Systematics

Characters (from Pellow et al. 2009): Herbs or small shrubs. Leaves toothed to lobed or dissected or rarely entire. Umbels usually compound, sometimes simple or reduced to a few or single flower. Compound umbels usually with 3-4 rays and bracts; umbellules with 2-3 bracts and several almost sessile flowers. Calyx lobes broad, orbicular to lanceolate. Petals usually with much inflexed apex. Fruit without vittae.

<==Xanthosia Rudge 1811 [incl. Leucolaena Brown 1814 (nom. illeg.); Xanthosieae]HH00
    |--*X. pilosa Rudge 1811 (see below for synonymy)HH00
    |--X. atkinsonianaHH00
    |--X. candidaGK00
    |--X. ciliataGK00
    |--X. dissectaHH00
    |--X. huegeliiHH00
    |--X. leiophyllaHH00
    |--X. scopulicola Hart & Henwood 2000HH00
    |--X. stellata Hart & Henwood 2000HH00
    |--X. tasmanica Domin 1907HH00
    |--X. ternifolia Hart & Henwood 2000 [=X. tasmanica Gandoger 1918 non Domin 1907]HH00
    `--X. tridentataHH00

*Xanthosia pilosa Rudge 1811 [=Leucolaena pilosa (Rudge) Bentham in Endlicher et al. 1837; incl. X. diffusa White 1942, X. pilosa var. glabra Moore & Betche 1893, X. hirsuta de Candolle 1830, X. p. var. longipes Domin 1907, X. montana de Candolle 1830, X. pilosa var. montana (de Candolle) Domin 1928, Leucolaena pannosa Bentham in Endlicher et al. 1837, X. pannosa (Bentham) Steudel 1841, X. pilosa var. pannosa (Bentham) Domin 1928, X. vestita Bentham 1867]HH00

*Type species of generic name indicated

References

[GK00] Gibson, N., & G. J. Keighery. 2000. Flora and vegetation of the Byenup-Muir reserve system, south-west Western Australia. CALMScience 3 (3): 323–402.

Harden, G. J. (ed.) 1992. Flora of New South Wales vol. 3. New South Wales University Press.

[HH00] Hart, J. M., & M. J. Henwood. 2000. Systematics of the Xanthosia pilosa complex (Apiaceae: Hydrocotyloideae). Australian Systematic Botany 13 (2): 245–266.

Liu, M., G. M. Plunkett, P. P. Lowry, II, B.-E. Van Wyk, P. M. Tilney & A. N. Nicolas. 2016. The phylogenetic significance of fruit and trichome structures in Apiaceae subfamily Mackinlayoideae. Systematic Botany 41 (3): 685–699.

Pellow, B. J., M. J. Henwood & R. C. Carolin. 2009. Flora of the Sydney Region 5th ed. Sydney University Press.

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