One out of the boxes

The Plant of the Month for October 2016 is Eucalyptus porosa F.Muell. ex Miq. (mallee box), the dominant eucalypt in DEWNR’s Park of the Month, Cobbler Creek Recreation Park.

Mallee box, Eucalyptus porosa, dominates the hillsides of Cobbler Creek Recreation Park, with red gum, E. camaldulensis, growing along the creekline. Photo: Peripitus (CC BY-SA 4.0) from Wikipedia.

Mallee box is part box (a general name for a Eucalypt with rough flaky bark) and part mallee due to its well-developed lignotuber and frequent multi-stemmed habit. Under optimal conditions though, it is often a single-trunked, rather twisty and spreading tree. Mallee box can be distinguished from other boxes in SA by the colour of its foliage: a fresh “yellow-ochre green” rather than grey-green.

Eucalyptus porosa foliage. Photo: Clive M. Chesson.

Eucalyptus porosa mostly occurs within South Australia, and although sometimes overlooked, it is a very characteristic feature of many of our landscapes, particularly on the sheet limestone soils of Eyre and Yorke Peninsulas. It also grows in clay-loam depressions in the Murray Mallee and upper South-East regions, on rocky hillslopes in the Flinders Ranges, and confined along drainage lines in more arid areas. In the Adelaide region it is localized on dryer sites of the hills-face escarpment from Cobblers Creek to Skye and further south near Marino. Several remnant trees also persist on the edge of Adelaide city in the West Terrace Cemetery.

Eucalyptus porosa flowers, buds and fruit. Photo: Clive M. Chesson

The variety of its habitats has probably contributed to the plethora of common names: mallee box, black (or South Australian) mallee box, water (or Quorn or lerp) mallee and swamp box.

Brooker (2000) classified Eucalyptus porosa as being closely related to other boxes such as peppermint box (Eucalyptus odorata) and grey box (E. microcarpa). However, recent unpublished molecular studies by Herbarium Molecular Botanist Ed Biffin show that it is most closely related to the South Australian blue gum (E. leucoxylon) in a different group comprising species that are remarkably diverse in their bark type – smooth-barked gums, boxes and ironbarks. Significantly, hybrids between E. porosa and E. leucoxylon are frequently encountered.

Descriptions of these and other South Australian eucalypts are available in the new Flora of South Australia (5th edition) eucalypt treatment (32MB pdf file) by Dean Nicolle.

Contributed by State Herbarium botanists Peter J. Lang.

Laver – a delicate red alga favoured as the wrapping around rice for sushi has appeared in a distinct, dark band of growth on boulders of the artificial breakwater of the Sea Rescue Marina(Barcoo Inlet), near West Beach, Adelaide.

Growth zone of Porphyra on a boulder at West Beach, South Australia. Photo: B. Baldock.

The species is Porphyra lucasii Levring. Uninspiring at first glance, a sheer, purple, frilly blade about 50 mm long can be seen if the alga is floated out in water.

Porphyra lucasii. Photo: B. Baldock.

It is more spectacular under the microscope – one cell thick with dazzling patterns of cells and a toothed blade edge. It clings to the rocks by a patch of cells (a “holdfast”) with snake-like projections that squeeze into minute crevices in the rock surface.

In southern seas, we see Porphyra only in winter. At the State Herbarium of South Australia, we have metropolitan Adelaide specimens from Port Stanvac and Brighton jetty piles, and Witton Bluff, Port Noarlunga.

Pophyra lucasii. Microscopic view of a blade edge (top) and holdfast cells with thread-like “tails” (bottom). Photo: B. Baldock.

But Porphyra has a secret life, not yet detected locally in the wild: a microscopic, thread-like spore phase. This is called a Conchocelis stage and gets its name because the tips of its threads penetrate shells of molluscs (“concho” meaning something to do with shells). – The discovery of this lifecycle in another species of Porphyra by British phycologist Kathleen Mary Drew-Baker in 1949 revolutionised seaweed production in Japan.

An unusual feature of the West Beach plants is their high position in the intertidal – merely in the “splash” zone, or “supra-littoral”, above high tide. Do they survive because seas have been so rough recently and they occasionally get splashed? In some places, the plants sit on a mat of seagrass fibres about 5 mm thick. Could this act like a sponge and sustain the alga with water at low tide and during calmer conditions?

Unfortunately, the band will disappear with the coming of summer, the conditions drying and shrivelling the delicate plants. Next winter, will they return?

Contributed by Bob Baldock and Carolyn Ricci, Phycology Lab., State Herbarium.