Time capsules: Australia’s remarkable native seeds have an ancient and intriguing legacy
If you want to understand the deep-past of a place, take a look at its seeds.
Australia, for example, is home to a stunning variety of seeds. Some are robust, almost unbreakable. Others seem impossibly fragile. There are enormous seedpods, spiked and forbidding, many weighty smooth-coated nuts, and seeds so insubstantial they resemble dust motes. Some seeds sleep well, woken reliably by gentle warmth and soft rain, as others wait patiently for fire. There are seeds that disperse thousands of kilometres or just a few inches. Some seeds endure only a handful of days while others traverse decades, easily outlasting a human lifespan or several.
The story of Australia’s seeds and their astonishing diversity begins with space and time. Specifically, the Australian continent takes up a lot of space — 5 per cent of the entire planet’s land mass, in fact — stretching across several climate zones from the northern tropics to the cool temperate south. Add to this a varied landscape of coasts, plains, mountain ranges and a vast centre sculpted by ancient seas. Given 40–50 million years of isolation from the rest of the world and the result is an ecological diversity quite unlike anywhere else. It should be little wonder that so many unique species arose, adapted and even thrived in Australia’s many ecological niches. Yet, it remains wondrous.
Today Australia’s biota accounts for 10 per cent of living species globally, a tally that includes more than 21,000 native Australian plants. Native mangroves skirt coastal waterways, while around one thousand native grass species sweep across the continent. There are also grass trees that are neither grasses nor trees. The Sturt’s desert pea grows in formidably arid conditions from the continent’s centre to its west, while golden wattles spill across the cool, damp southeast.
Australia is also home to a rich collection of Myrtaceae, the plant family to which myrtles and eucalypts belong. Indeed, most of the world’s 800 species of eucalypts are endemic only to Australia where they dominate the landscape. Meanwhile, there are many rare plants just clinging to existence – a lonely grove of Wollemi pines here, a final cluster of Grampian pincushion lilies there. Their purpose, as with any species, is to simply endure so long as they can, which brings us to the matter of seeds.
Not all Australian plants produce seeds, but most of them do. If a seed plant is to pass on its genes to the next generation — its seeds must be up to the task. What’s particularly compelling is the diversity of ways they contrived to achieve this, even when they belong to the same plant family.
Take for example, all those Myrtaceae, many of which evolved as Australia’s climate dried out over millions of years. As a result, most produce seeds capable of enduring dry conditions. This is why many eucalypts produce dry, woody fruits full of what botanists call ‘orthodox’ seeds, which tolerate the drying and cooling process required for long-term seed storage. In fact, the longest-lived seeds of any Australian species are produced by the mouse ears bush (Calothamnus rupestris), a hardy Myrtaceae endemic to Western Australia.
Then there is the curious case of the shy feather flower (Verticordia fimbrilepis subsp. Fimbrilepis), an endangered Myrtaceae subspecies from the same region. It’s a stunning little shrub with bright pink flowers and long-lived seeds that germinate particularly well following a bushfire. It joins a long list of plants called ‘fire followers’ or ‘fire ephemerals’ from a multitude of different Australian plant families, which produce seeds that germinate after fire. These seeds contain echoes of adaptations played out over eons on an increasingly fiery continent. Sometimes, the heat of a fire helps to release seeds. In other cases, seeds lie in the soil for years waiting, not for the heat, but for specific chemical compounds produced by burning plants. By germinating soon after a fire, the next generation has a chance to flourish in the presence of fresh post-fire nutrients while the probability of the next fire is still low.
Of course, there are many Australian seeds that require almost constant moisture. Such is the case with endemic rainforest lilly pillies (Syzygium species). Though belonging to Myrtaceae, these plants produce large fleshy fruits and the seeds they contain often sprout soon after they land on the forest floor. Some even germinate while still on the tree. These seeds are ‘recalcitrant’ — they cannot tolerate drying or cooling and don’t live very long – a common trait among the seeds of fruiting rainforest trees. Although Australia’s rainforests contracted as the continent dried out, they never disappeared, so it was neither necessary nor strategic for seeds in those forests to adapt to protracted drought. Meanwhile, fleshy moisture-rich fruits remained a good way to entice seed dispersal by animals.
And so Myrtaceae provide us with an enticing glimpse of the many ways in which Australian seed plants adapted to myriad biomes, negotiating evolutionary trade-offs uniquely suited to a vast and varied landscape. Indeed, every seed is a time capsule, in more ways than one. Each provides a way for a plant to cast its genes forward into the future and, in every native seed, there is also a story of the land itself.
Fiona McMillan-Webster is a science writer based in Brisbane. She is the author of the book The Age of Seeds: How Plants Hacked Time and Why Our Future Depends on It.