Xylomelum

''Xylomelum'' is a genus of six species of flowering plants, often commonly known as woody pears, in the family Proteaceae and are endemic to Australia. Plants in this genus are tall shrubs or small trees with leaves arranged in opposite pairs, relatively small flowers arranged in spike-like groups, and the fruit a woody, more or less pear-shaped follicle.

Description

Plants in the genus ''Xylomelum'' are shrubs or trees that typically grow to a height of and have simple, leathery leaves arranged in opposite pairs. Juvenile leaves have coarse, sometimes prickly teeth on the edges. The flowers are arranged in dense, spike-like racemes or panicles, the flowers in pairs with a bract at the base. Each flower has four similar tepals that roll back as the flower develops revealing four stamens. The fruit is a woody, more or less pear-shaped follicle attached at the larger end, that eventually splits into two halves and releases two winged seeds.

Taxonomy

The genus ''Xylomelum'' was first formally described in 1798 by James Edward Smith in ''Transactions of the Linnean Society of London''. The name ''Xylomelum'' is derived from the Greek ''xylon'' meaning "wood" and ''melon'' "tree-fruit", referring to the woody fruit.

In 1979, Farr, Leussink and Stafleu nominated ''Xylomelum pyriforme'' (Gaertn.) R.Br. as the lectotype, later corrected to ''Xylomelum pyriforme'' (Gaert.) Knight.

Evolution

The "pears" of ''Xylomelum'' species are thought to have evolved primarily as a defense against seed predation. However, these woody fruits also provide protection of the seeds from fire. Johnson and Briggs (1963) consider the woody seeds in Proteaceae to be a late evolutionary adaptation to fire.Johnson, L.A.S. & Briggs, B.G. 1963. Evolution in the Proteaceae, ''Australian Journal of Botany'', 11:21-61

Species list

The following is a list of species accepted by the Australian Plant Census as at November 2020:
* '' Xylomelum angustifolium'' Meisn. (W.A.)
* '' Xylomelum benthamii'' Orchard (Qld.)
* '' Xylomelum cunninghamianum'' Foreman (Qld., N.S.W.)
* '' Xylomelum occidentale'' R.Br. (W.A.)
* '' Xylomelum pyriforme'' ( Gaertn.) Knight (N.S.W.)
* '' Xylomelum scottianum'' ( F.Muell.) F.Muell. (Qld.)

A seventh species, ''Xylomelum salicinum'' A.Cunn. ex R.Br. is listed at Plants of the World Online but is regarded as a synonym of '' X. scottianum'' by the Australian Plant Census. In 2007, Anthony Edward Orchard suggested that ''X. salicinum'' (Meisn.) Benth. is a superfluous, illegitimate name and raised ''X. pyriforme'' var. ''salicinum'' Meisn. to species status as '' X. benthamii''.

In 1788, Joseph Gaertner described ''Banksia pyriformis'' in '' De Fructibus et Seminibus Plantarum'', a species whose genus was effectively reclassified when Smith described the genus, ''Xylomelum'' in 1798. Smith gave the essential characters of ''Xylomelum'' as: flower spikes with simple scales; flowers with four petals bearing four stamens, with a blunt, club-shaped stigma, with a uni-locular capsule having two seeds, the seeds being winged.

In 1810, Robert Brown gave a key to the genera of Proteaceae in ''Transactions of the Linnean Society of London'', placing ''Xylomelum'' in the group of Proteaceae which have dehiscent fruit and are unilocular with two seeds, thereby grouping '' Anadenia'', '' Grevillea'', ''Hakea'', '' Lambertia'', '' Orites'' and '' Rhopala'' together. Within the group, ''Xylomelum'' is distinguished from '' Orites'' and '' Rhopala'' by having winged seeds, a thickened woody follicle with an excentric locule, and a club-shaped stigma.

In 1888, Engler, in '' Die Natürlichen Pflanzenfamilien'', divided the Proteaceae into two sub-families, Persoonioideae and Grevilloideae, placing ''Xylomelum'' in Grevilloideae, and as being like '' Helicia'' in having the flower axis at the base with four glands, but differing from it by having the inflorescence in 'ears', not in loose 'grapes', and by having seeds with wings.

In 1975, Johnson and Briggs placed ''Xylomelum'' in the subfamily Grevilleoideae and the tribe Heliciaea (together with '' Helicia'', '' Triunia'' and '' Hollandaea'').Johnson, L.A.S. & Briggs, B.G. 1975. On the Proteaceae-the evolution and classification, ''Botanical Journal of the Linnean Society'' 70(2): 83-182 However, Weston and Barker (2006), argue that within the tribe of Roupaleae, ''Xylomelum'' should be placed in the subtribe of Lambertiinae and not together with the '' Helicia'' in the subtribe of Heliciinae, using in part, the evidence of the molecular studies of Hoot and Douglas,Hoot, S.B. and Douglas, A.W. 1998. Phylogeny of Proteaceae based on atpB and atpB-rbcL intergenic spacer region sequences. ''Australian Systematic Botany'' 11: 301-320 who pair ''Xylomelum'' with '' Lambertia''.

See Grevilleoideae for further discussion of the placement of ''Xylomelum'' within the Proteaceae.

Distribution and habitat

Two species, '' X. angustifolium'' and '' X. occidentale'', are endemic to Western Australia and the remaining five species are found in New South Wales and Queensland. All are either mesophytes or grow in dry environments.

References

{{Taxonbar, from=Q2707698

Proteaceae genera
Proteales of Australia
Trees of Australia
Taxa named by James Edward Smith
Endemic flora of Australia