Friday, February 1, 2013

Portulaca intraterranea - A "Desert Gem"

Portulaca intraterranea J. M. Black is a very attractive species from the more arid parts of tropical and subtropical Australia. It is similar in many respects to P. oleracea and the two species may be difficult to distinguish when not in flower. It is most often found on red sand hills in the arid centre of Australia. However it has also been found on coastal sand dunes and sandy river flats in the tropics, as far north as Cape York. Some of the northern forms may prove to be distinct taxa.

Portulaca intraterranea, growing on a red sand dune in the far
west of New South Wales. This is the only photo I have on file of this
spectacular species. Somebody sent this photo to me many years ago
 and the source details have been lost. I believe it may have been copied from
 a book.  If someone owns the copyright on the photograph or does not
wish me to use it, please send me a message or post a comment below.

P. intraterranea may sometimes be confused with several of the exotic cultivars found in Australian gardens, such as P. umbraticola and P. cryptopetala. These come in a range of colours including yellow. But the resemblance is superficial. I intend to write about these introduced garden species in a later post.

P. intraterranea differs from P. oleracea by the following characters:
  • flowers in 3-4 flowered heads (cf. 2-30 for P. oleracea), 
  • significantly larger petals (10-17 mm long, cf. 4-7 mm for P. oleracea), 
  • petals twice as long as the sepals (cf. scarsely longer than the sepals for P. oleracea),  
  • greater number of stamens (more than 20 for P. intraterranea, cf. less than 20 for P. oleracea),
  • a generally stouter growth habit (cf. sprawling and spreading habit for P. oleracea).
In taxonomic illustrations, P. intraterranea is often shown with cuneate-oblanceolate ("wedge-shaped") leaves while P. oleracea tends to have distinctly obovate leaves. However this appears to be only a general rule.

I have seen photos of pressed herbarium specimens with flowers around 35 mm in diameter. Such plants must have been quite spectacular in the fresh state.

The seed is distinctly comma-shaped, grey-black in colour, sometimes shiny (or iridescent). The testa cells are basally stellate and forming prominent tubercles in ornate, concentric rows which are nippled or pointed. PlantNET illustrates these characters well.

By comparison, the seed of P. oleracea is shiny black and more rounded in outline. The testa is only  minutely tuberculate, the concentric rows are less distinct, and the testa cells are rarely pointed but more often rounded.

Some authors claim that P. intraterranea has a distinct taproot, but other authors do not mention this. It would also appear from the literature that there are both annual and perennial forms.

As is the case with P. oleracea, the species P. intraterranea is morphologically variable over its range of distribution. Some of the variants may prove to be distinct taxa.

Portulaca sp. affin intraterranea (or sp. affin. australis?).
This interesting species was photographed on Cape York.
When not in flower it resembles P. australis, except for the more
spreading branches. Photos courtesy of Jill Newland and Roger Fryer.

 Above: The capsules are domed and dehisce about mid-way.
 This character is typical of P. australis.
Below: Some of the axillary hairs can be quite long, however
P. intraterranea generally has hairs only 1-2 mm long.

Below The greatly exserted stigma rules out P. australis
immediately. The plant will key to P. intraterranea, but is
atypical in many ways. It is probably a distinct taxon.

Indeterminate taxa are often encountered on inland sand or clay plains. One has quite small, oblong leaves, a compact growth habit, flowers 8-12 mm and a stamen count of 10-20. It looks like a larger-flowered form of P. oleracea. Yet it is recognizable by sight and may even co-habit areas with more typical forms of P. oleracea. Other forms are indistinguishable from P. oleracea, apart from the much larger flowers. Many of these plants appear to fit broadly into a pan-Pacific P. lutea clade, which includes P. howellii D. Legrand from the Galapagos Islands.

Portulaca sp. affin. intraterranea.
This plant from a sandy clay plain near Cloncurry appears to be
 intermediate between P. oleracea and P. intraterranea. Such plants
are widespread yet can be exceedingly difficult to identify.
The photo was sent to me by Sally-Ann Barrs, an environmental
officer working for Ernest Henry Mining Pty. Ltd. in Cloncurry.
The plant was officially identified as P. intraterranea.

The author Attila Kapitany has kindly sent me a CD of his photographs of P. oleracea and other species. Some of the species were photographed in habitat, others were grown from seed in his Melbourne garden. I have tried, but in many cases failed, to sort the photographs in the "Oleracea" folder into identifiable species. I will include some of the photographs here. One plant shows a flower with 20+ stamens, but the rest of the flowers on the same plant barely have 10 stamens. Yet the flowers are noticeably larger than typical forms of P. oleracea. I am sure that Australian herbaria must be full of specimens like these. In some cases botanists may have made a "best judgement" based on the available characters. In other cases the specimens may have been deemed indeterminate. It is possible that seed may go some way towards providing a positive identification in these cases.

Here are some of the photos. You be the judge!

This flower (above) has multiple stamens and petals around 1 cm,
 however the flowers (below) are similarly-sized with only
 about 10 stamens. Both photos are of the same plant!
So,  is the plant a form of P. oleracea or P. intraterranea?
(Note: The hairy, terete-leafed plants also in frame are P. pilosa.)

The rather tardy, compact plant (above) is from a desert area. It
may be a form of P. intraterranea. It is growing on stabilized
gravel that has been coated with a thick layer of "desert varnish".
Photo courtesy of Attila Kapitany.

Above: This cultivated plant may be a form of P. intraterranea
 or an undescribed taxon allied to it (or P. oleracea). 
Note: The terete-leafed plants also in frame are P. pilosa.

Below: A very similar plant with quite showy flowers.
Bottom: The same plant showing flowers larger than P. pilosa.

Photos courtesy of Attila Kapitany.

I am constantly surprised by the apparent lack of interest in P. intraterranea in Australia. The species is described in dry, technical terms in official and scientific literature, but has been largely ignored by horticultural journals. Equally surprising is the chronic lack of photographs of the species! The few that I have found online appear to be either misidentifications of P. oleracea or are one of the "indeterminate" forms mentioned in the previous two paragraphs.

The species has clear horticultural potential, yet it would appear that no work has been undertaken with a view to producing select cultivars. The outcrossing of selected cultivars, especially perennial forms, may produce larger-flowered clones with increased vigour. There is scope to produce plants of equal callibre to the exotic P. umbraticola and P. grandiflora cultivars that are so commonly grown in summer gardens in Australia.

This species clearly deserves more attention from both science and horticulture.

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