Hansjörg Eichler Scientific Research Fund – Report
Leaf blades or floral clades - A guide to spinifex
Jim Mant, Australian National University
The Australian tribe of grasses, Triodieae or 'spinifex', was well known
to the early explorers of the continent's centre. The tough sharpened
leaf blades of this 'porcupine grass' were a dreaded obstacle to the adventurers'
vain pursuit of fertile lands and inland seas - a real Australian 'thorn
in the side'. For most, the endless spinifex grasslands symbolised a harsh
and monotonous, if not hostile Australian desert, a place to heroically
endure rather than celebrate.
Yet, spinifex grasses are of great importance to what is a dynamic, biologically
diverse and culturally rich arid zone. In many areas these grasses form
the primary perennial biomass, providing habitat for the world's richest
lizard fauna and several endangered mammals, playing a dominant role in
the region's fire ecology and supporting a diverse ephemeral flora in
inter-hummock areas (Griffin 1984). They form pure hummock grasslands
or a dominant understorey on over two million km2 of the continent
- the single most extensive vegetation type in Australia (Griffin 1984).
William Dampier, in 1699, was the first European to collect a hummock
grass (George 1999), although indigenous Australians have long used and
traded the adhesive resin produced by some spinifex species. Dampier collected
a specimen of what is now called Triodia danthonioides, from the
southwest coast of Western Australia. For many years, the species was
included in the genus Plectrachne, along with 15 others with remarkably
similar spikelet characters of long linear glumes and three elongated
awns on the lemma.
In contrast, Triodia, described by Robert Brown, has traditionally
been distinguished from Plectrachne by its relatively short obtuse
glumes and variously lobed or emarginate lemmas. In his recent comprehensive
revision Mike Lazarides (1997) transferred Plectrachne into Triodia; the original glume and lemma criteria failed to account for the many
species newly described in the postwar period. Instead, nine infra generic
groups were erected similarly based on features of the spikelet and inflorescence.
A couple of smaller genera were retained; Monodia which is monotypic
and Symplectrodia with two species from Arnhem Land.
The tribe now comprises 67 described species and includes considerable
variation in the floral parts. However, there appear to be two quite different
types of leaves that cut across these generic and infrageneric classification
schemes. This leaf anatomical variation was first documented by Nancy
Burbidge in the 1940's, but is significant enough to have attracted the
common names, 'hard' versus 'soft' spinifex. 'Hard' taxa have stomata
on both sides of the leaf, whereas 'soft' lack stomata and associated
photosynthetic tissues from the outer leaf surface. 'Soft' spinifex species
are, with one exception, restricted to the monsoonal region of the arid
north. Species with the 'hard' type anatomy are found throughout Australia's
north as well as the temperate south (Lazarides, 1997).
A survey of the 67 described species in the tribe identified 40 with
the 'hard' type anatomy including T. danthonioides, whilst 27 species
are 'soft' (Mant, unpublished). Both types occurred in the former Plectrachne and Triodia, and now six of the infrageneric groups are polymorphic
for the leaf character. Monodia is 'soft', while Symplectrodia is 'hard'. Have these leaf forms arisen many times as the classification
implies? Or are there, instead, discernible patterns of convergence in
To resolve these competing claims for character importance, a morphological
and molecular phylogeny of the tribe was attempted with some much needed
financial assistance from the Hansjörg Eichler Research Fund. Molecular
results from the ITS region of nrDNA support the recognition of two lineages
corresponding to the hard - soft leaf anatomy, as well as confirming the
tribe's monophyly (see Mant et al. in press). Symplectrodia is nested within the 'hard' group, while the relationship of Monodia to the 'soft' clade remains uncertain. Instead of plasticity in leaf
anatomical characters, these data point to strong convergence in key spikelet
and inflorescence characters. It would seem that both Plectrachne and Triodia were not monophyletic as previously circumscribed.
Outgrouping supports a single derived origin of the 'soft' leaf, with
the 'hard' type species forming a basal paraphyletic group in the tribe.
Re-examination of the leaf anatomy indicates the two forms are likely
to have markedly different physiologies relating to water use and photosynthetic
activity (McWilliam and Mison 1974; Craig and Goodchild 1977). The 'soft'-type
leaf anatomy may well be an adaptation to the more predictable rainfall
of the monsoonal arid north.
In any case, Dampier's T. danthonioides and its other 'hard'
allies from S-W Western Australia do not share recent ancestry with the
northern monsoonal 'soft' species such as T. schinzii (the
type of Plectrachne) and T. pungens (the type of Triodia). Overall, I would argue that features of the spikelet
and inflorescence are unsuitable for generic classification in this tribe
of grasses. Instead, it turns out that the key to the thorny issue of
higher level spinifex relationships is to be found within those harsh
spiky desert leaves.
Craig, S. and Goodchild, D. J. (1977) Leaf Ultrastructure of Triodia
irritans: a C4 grass possessing an unusual arrangement of photosynthetic
tissues. Australian Journal of Botany 25: 277-290
George, A. S. (1999) William Dampier in New Holland. Australia's First
Natural Historian. Melbourne: Bloomings Books.
Griffin, G. F. (1984) Hummock Grasslands. In G. N. Harrington, A. D.
Wilson and D. Young, eds. 'Management of Australia's Rangelands'. pp.
271-284. CSIRO: Melbourne.
Lazarides, M. (1997) A revision of Triodia, including Plectrachne (Poaceae, Eragrostideae, Triodiinae). Australian Systematic Botany 10: 381-489
Mant, J. G., Bayer, R. J., Crisp, M. D. and Trueman, J. W. H. (in press)
A phylogeny of Triodieae (Poaceae: Chloridoideae) based on the ITS region
of nrDNA: testing conflict between anatomical and inflorescence characters.
In: S. W. L. Jacobs and J. Everett, eds. Grasses - Systematics and
Evolution - Vol. 2 of Proceedings of the Second International
Conference of the Comparative Biology of the Monocots. Sydney:
McWilliam, J. R. and Mison, K. (1974) Significance of the C4 pathway
in Triodia irritans (Spinifex), a grass adapted to arid environments. Australian Journal of Plant Physiology 1: 171-175
Jim Mant studied the systematics of Triodieae for his BSc Honours degree
at The Australian National University and CSIRO Plant Industry. He is
now doing a PhD on the comparative biology of Chiloglottis (Orchidaceae)
and their sexually deceived thynnine wasp pollinators at ANU and Royal
Botanic Gardens Sydney.
Published in Australian Systematic Botany Society
Newsletter 102; 5-6 (March 2000)