Thursday, December 27, 2012

Portulaca australis

A particularly showy specimen of P. australis.
Photo courtesy of Attila Kapitany.

Portulaca australis Endl. is a very widespread species in Queensland but is only occasionally encountered. It can however be locally common in the right kind of habitat, especially in the summer and autumn months following good rainfall. The species has a preference for granite rock platforms in Eucalypt forest, where it frequently occupies narrow cracks and crevices that have accumulated some gravel and humus. Here the plants are often found in the company of lichens and mosses.

Sloping granite rock platforms in Eucalypt forest provide a 
suitable habitat for P. australis and other Portulacaceae.
Photo courtesy of Attila Kapitany.

Sometimes P. australis can be found around the edges of exposed boulders, particularly along ephemeral seepage lines on well-drained sites. It has also been found less commonly on flat, inland plains of lower rainfall, where skeletal soils or gravels overlay hard claypans. In this situation the clay probably acts as a pavement-like barrier which greatly enhances drainage. In the tropics it has occasionally been found close to tidal sand flats and on sand dunes. In all situations sun exposure is generally high, although some shade may occur as a result of boulders, trees, shrubs, grasses, and aspect.

The leaves of P. australis are crowded, especially on juvenile specimens. Leaves are often linear but slightly flattened on young plants, but become more oblong-elliptic with age. The leaves on mature plants are only about 4-10 mm long X 1-4.5 mm wide and have a sub-obtuse apex (i.e slightly rounded off, not quite pointed) with a truncate to tapered base.

Juvenile plants of P. australis (left) and P. pilosa (right) frequently
emerge from crevices in rocks. The specimen at left is growing amidst
lichen-encrusted basalt rocks. The specimen at right has turned red due to high
sun exposure. Photo courtesy of Attila Kapitany.

The species P. australis is similar to P. filifolia Muell., a very widespread native species with sparser, more terete leaves and an upright or diffuse growth habit. P. filifolia is generally found on flat sandy plains in inland areas.

P. australis differs from P. filifolia by having shorter, more crowded leaves, (cf. 10-30 mm long for P. filifolia), a generally more prostrate growth habit, shorter petals (5-6 mm compared to 8-10 mm for P. filifolia), a stigma shorter than the stamens (cf. longer than the stamens for P. filifolia), stigmatic lobes numbering 5-6 (cf. 3-5 for P. filifolia), sepals less than 4 mm long (cf. 4-5 mm long for P. filifolia), and its elongated capsules circumcissing at the middle (cf. below middle for P. filifolia). Generally speaking the flower petals are more rounded for P. australis, but this may not always be the case. Plants of P. australis are generally prostrate, but may become slightly upright in times of high humidity and cloud cover. Juvenile plants of P. australis, P. pilosa, and P. filifolia can be very difficult to tell apart, even if grown side by side, as all three species tend to be prostrate at this time.

P. australis is generally prostrate, especially during dry
conditions (above), but plants may become upright in
shade or during humid, cloudy weather (below). 

Photos courtesy of Attila Kapitany.

P. australis is said to have smaller flowers generally than P. filifolia, but some forms can have quite showy flowers up to about 10 mm in diameter. The flower colour can range from lemon yellow to almost orange.  

P. australis flowers typically have the stigmatic lobes resting
within the stamens and generally not rising above them.
Photos courtesy of Attila Kapitany.

A particularly large specimen of P. australis, which almost appears
to be forming the shape of Australia! This is a cultivated specimen.
Plants in habitat seldom attain this size.

 P. australis typically has a tuberous rootstock which enables plants to survive long periods of drought. Although it is generally regarded as an annual or short-lived perennial, many of the plants I have observed in the wild and in cultivation are long-lived. In drought times and during the cold winters, they will die back almost to ground level leaving just a few very wilted leaves. Plants resprout from a small stem at the top of the tuber year after year. One plant in cultivation lived for about 8 years.

Thickened taproots and secondary tubers of P. australis,
 which enable the species to survive periods of drought.
Photo courtesy of Attila Kapitany.

Footnotes: I have encountered specimens that completely lack a tuber, both in their early stages of growth and at maturity. The flowers are typically larger and often have an orange tint on the petals. Some plants have even had a splash of light red in the centre. Although they will key out to P. australis, I am unsure if these plants are still within the range of variation of P. australis or if they represent a distinct but undescribed taxon. It is also possible that they have resulted from chance vegetative propagation of otherwise tuberous plants.

Although it seems convenient to predict that tuberous plants will all be perennials while fibrous-rooted plants will all be annuals, my observations of wild and cultivated plants suggests that this is not always the case. I have noted that some predominately fibrous-rooted plants have re-emerged from the same rootstock each spring/summer while some predominately tuberous plants have not re-emerged the following year. The variable root structure of some native Portulaca species greatly complicates their taxonomy. Further research is needed to determine if root character is fixed in some populations or individuals, or if it is triggered by environment.

There is anecdotal support for the existence of a yellow-flowered form of P. pilosa in America and Australia, but it is important to remember that P. pilosa always has free stamens while P. australis has basally connate stamens (i.e. the bases of the filaments are organically fused together). The difference is clearly illustrated in the following photo.

P. australis (yellow) growing with P. pilosa (purple).
 P. australis has basally fused filaments and
stigmatic lobes positioned below the anthers.
P. pilosa has free filaments and stigmatic lobes
exserted above the anthers. Photo by Attila Kapitany.

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