Passing awns
Published 13 May 2023
The dryland spinifex-grasses are a uniquely Australian affair, creating a landscape unlike any found elsewhere. Their classification, however, has not been free of contention. And one of the biggest questions about placing the spinifexes has been what to make of Plectrachne.
The genus Plectrachne was first established by J. Th. Henrard in 1929 for one Australia’s most widespread and abundant spinifexes, P. schinzii. Subsequent authors expanded the genus until it covered some 18 species. Plectrachne was distinguished from the more diverse genus Triodia by two features. Firstly, the glumes (the pair of bracts at the base of a spikelet or flower cluster) were much longer than the individual florets. And secondly, the lemmas (the larger of the bracts subtending individual florets) were deeply divided into three lobes with each lobe tipped by a long needle-like awn (Lazarides 1997).
The problem with these distinctions is that, as time went by, the dividing line became less and less clear. Several species were assigned to Plectrachne in which the glumes were shorter than the florets. Several more in both Plectrachne and Triodia had variable-length glumes that might be shorter or longer depending on happenstance. Numerous species of Triodia had lemmas ending in teeth or lobes that differed from Plectrachne only in lacking awns. In the species Plectrachne bynoei, the division between the lemma lobes was relatively shallow and the awns fairly short. Because of this gradation in characters, Lazarides (1997) eventually just merged the two genera together, placing all the Australian spinifexes in the single genus Triodia.
This approach was reinforced by later molecular phylogenetic analysis which found the species of ‘Plectrachne’ to be scattered among Triodia rather than forming a single clade (Crisp et al. 2015). Both ‘Triodia’ and ‘Plectrachne’ included both resinous and non-resinous species, reflecting a failure of spikelet structure to correlate with growth habit. For now, Australian plant taxonomy has the spinifexes standing together.
References
Crisp, M. D., J. Mant, A. Toon & L. G. Cook. 2015. Australian spinifex grasses: new names in Triodia for Monodia and Symplectrodia. Phytotaxa 230 (3): 293–296.
Lazarides, M. 1997. A revision of Triodia including Plectrachne (Poaceae, Eragrostideae, Triodiinae). Australian Systematic Botany 10: 381–489.