Saturday, December 29, 2012

Portulaca filifolia

P. filifolia typically has an erect-diffuse habit, terminal
flowers on long, thin branches, and sparse, terete leaves.
Photo courtesy of Attila Kapitany.

Portulaca filifolia Muell. may at first glance appear similar to P. australis Endl., but the similarities are superficial. In any case they seldom occur together in the wild. The two plants in the following photograph were grown in cultivation for comparison purposes. The prostrate species at left is P. australis, and the more upright and diffuse species at right is P. filifolia. (Click on thumbnails to enlarge all photos.)

P. australis at left and P. filifolia at right.
Photo courtesy of Attila Kapitany.

P. filifolia is widespread in inland Australia. It generally occurs on flat sandy or stony plains, where it grows in full sun or under the sparse cover provided by trees and bushes. It occurs on sandy floodplains, near inland lagoons, and along ephemeral wash areas. It is often found where a shallow lens of light sand has been deposited over heavier clay. It frequently forms part of the sparse ground flora in cypress pine forests and poplar box woodlands.

The plant is generally regarded as an annual. It has an upright, reclining or diffuse growth habit, with stems to about 20 cm long. The leaves are linear and terete, 10-30 mm long and sparsely situated along the branches.

Above: Bright yellow flowers in terminal clusters on a rather
sparse and untidy plant typifies P. filifolia.
Photo courtesy of Attila Kapitany.

Below: Terminal leaves, flowers, and pyxis of P. filifolia.
Photo by Ian Menkins.

The axillary hairs are conspicuous and generally numerous, especially around the terminus of the branches, and are often more than 5 mm long. The hairs apparently suffer the effects of ablation due to wind and rain and possibly also the activities of birds, rodents and insects. As a result, some mature plants may appear to have few or very short axillary hairs. Flowers are terminal and in 3–10-flowered clusters. The petals are (4–)8-10 mm long and yellow, with 12–30 stamens. The sepals are 4-5 mm long. The stigma is longer than the stamens.

Above: P. filifolia, terete leaves and terminal, clustered flowers.

Below: Close up showing stigmatic lobes longer than the stamens.

Photos by Ian Menkins.

The capsules are 3–5 mm long, and the operculum (i.e. the "upper" dehiscing part of the capsule) is rounded or dome-shaped and about twice as long as the lower capsule. The operculum separates below the middle of the pyxis and often retains the withered remnants of the corolla.

P. filifolia, terminal cluster of capsules with one dehisced operculum.
Photo by Ian Menkins.

The seeds may be grey-black or shiny black and the testa cells are basally stellate and most are tuberculate. The tubercles are nippled and pointed and occur in ornate, concentric rows. In rare cases only the marginal or upper tubercles are pointed.

The shape and texture of the testa is illustrated on Plantnet - (Entry for P. filifolia). The following microscope captures are at 20X and 40X magnification and show the subtle variations in seed collected from a single population of P. filifolia. The seed are fairly consistent in size, shape and colour. The testa is for the most part rather black or grey-black in the fresh state, but soon turns steely-grey and somewhat iridescent upon exposure to air and sunlight. Some seeds are brownish due to a coating of fine dust particles. The bases of the tubercles are generally stellate. The tubercles range in shape from colliculate with rather low, oblong and flat summits to raised and prominently nippled. The tubercles are arranged in an ornate series of concentric rings. The margins of the seeds resemble the tread of a tyre if not for the prominent nipples!

This species may or may not have a rather soft, oblong, perpendicular, whitish or creamy taproot or tuber ca. 4 cm long and ca. 6 mm wide. Some secondary tubers may also occur, subtended by fibrous roots. Occasionally the tuber may have multiple constrictions, giving it the appearance of a string of small sausages.

The existence or absence of tubers is something that has baffled me for many years. Last year when I thinned out my excess P. filifolia plants from the sand bed, I noticed that almost every specimen had a tuber. Probably around 98% of the hundred or so plants that I removed had tubers. The tubers were more prevalent on juvenile plants. However, when I thinned out my plants this year I only found two plants with slightly swollen roots and they were mature plants! All of the other plants had only fibrous roots!
Above: P. filifolia plants in different stages of growth from the
summer of 2012-13. The majority of plants had fibrous roots.

Below: One of only two mature plants that showed
any evidence of taproot enlargement or tuber
formation this year, albeit insignificant.
Photos by Ian Menkins.

I can only conclude that the formation of greatly swollen taproots and tubers is not always a reliable trait in some species of Portulaca. It may be a purely adaptive feature that is triggered by the prevailing weather conditions. For instance, if the weather is dry and hot for many months, a survival gene may be triggered so that plants will develop storage tubers. In good seasons when moisture and nutrients are plentiful, the gene will not be triggered. In this case only fibrous roots will continue to grow. The substrate and drainage may also play a part. The contradiction of tuberous and non-tuberous plants has also been noted when studying P. australis. The same explanation may apply, but this interesting feature clearly demands more study. For instance, could Mycorrhizal fungi be involved?

Footnote: Plants in tropical regions that consistently have longer leaves and petals 10-16 mm long may represent P. decipiens (von Poelln.), although the species is currently placed in synonymy with P. filifolia Muell. I have discussed this taxon in the earlier post Problems in Australian Taxonomy and intend to discuss it further in later posts.

A large clump of the naturalized P. pilosa in foreground,
with the native species P. filifolia growing behind it.
Photo courtesy of Attila Kapitany.

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