The immortals

Humans in industrialised nations typically live around 80 years before succumbing to cardiovascular disease, cancer or other diseases of ageing.

The oldest human on record, Frenchwoman Jeanne Calment, died of heart failure in 1997, aged 122. More commonly, cancer is the ultimate price of human longevity: new research supports a hypothesis that many tumours and leukaemias arise from mutations in "immortal", pluripotent stem cells that constantly renew the body's tissues and organs.

Even by the measure of Jeanne Calment's extreme longevity, some woody plants, both conifers and flowering plants, are virtually immortal. They can continue to regenerate from meristem cells - the plant equivalent of stem cells - for millennia, without accumulating genetic errors, or plant tumours. How?

In 1937 Tasmanian bushman Denny King discovered a new species of lomatia, Lomatia tasmanica, (now popularly known as King's holly), a distant cousin of the waratah, in the island's perennially wet south-west.

The shrubby plants form a linear population that wends its way through more than 1.6 kilometres of dense temperate rainforest.

Cytogenetic analysis revealed they are genetically identical: they are all clones of a rare, triploid progenitor whose dull pinkish red flowers - unique in the trans-Pacific genus - are sterile and set no seed.

Morphologically identical, sub-fossilised leaves found in late Pleistocene sediments 8.2km away have been radiocarbon-dated at 43,000 years - not coincidentally, the practical limit of radiocarbon dating, so they could even be older. So King's Holly is at least 10 times older than "Methuselah", a 4773-year old bristlecone pine (Pinus longaeva) in California's White Mountains.

Methusaleh's status as the world's oldest tree is contestable. On the exposed, rain-swept upper slopes of Mt Read, near Rosebery in western Tasmania, grows a one-hectare stand of gnarled, stunted Huon Pines (Lagarostrobus franklinii).

Some larger trees in a sheltered grove are more than 2000 years old. All trees are genetically identical, and male - a clone.

No other Huon grows within 20km of Mt Read. Pollen from the sediments of a glacial tarn, Lake Johnston, downslope of the krumholtz ('twisted wood') pine, is of the same genotype. It has been radiocarbon-dated at 10,500 years.

But even Mt Read's venerable Huon pine may be younger than the diminutive Mongarlowe mallee, Eucalyptus recurva, discovered by an amateur botanist near Braidwood, in south-eastern NSW.

There are only four tiny, isolated populations, all apparently clones of long-vanished original trees that survived the last glacial period. The small leaves are the thickest of any eucalypt, and are studded with large oil glands - adaptations to killing frosts.

Unlike Kings Holly and the Mt Read Huon pine, the Mongarlowe Mallee sets seed, but produces no new seedlings. They germinate only after being refrigerated for months in the laboratory, at freezing temperatures. Since the last glacial period 12,900 years ago, winters may not have been cold enough - nor long enough - to promote germination.

Peter Waterhouse's research into the mysterious mechanism of spreading RNA interference hints at the existence of an RNA-mediated mechanism that insulates meristem cells in the growth shoots or epicormic buds of these "immortals" against time and pathogenic tide, so they continue to replica with extraordinary fidelity.