The continent of Australia broke off from the supercontinent Gondwana about 55 million years ago, so its plants evolved in isolation from the other land masses. It is now largely desert with smaller areas of temperate and alpine climates and a substantial tropical rainforest on its northern coast. Australia has the least geologic activity of all the continents, which has consequences to the adaptation of plants.
Adaptation to Soil
Glaciers and the impact of freezing and thawing water on other continents have produced new soils over millions of years, some of which are rich black in color. Australian soils are largely produced by the oxidation of iron oxides, yielding red soil that is blown by the wind or leached by occasional heavy floods. Many plants in Australia have adapted to these conditions by developing special roots that allow them to cling to rocks and survive the lack of moisture in deserts.
The wind, bees and other insects pollinate plants outside of Australia. Wind can spread pollen when trees are close together, but the many widespread plants in Australia have had to adapt. Australian flowers have huge blossoms to support pollinating birds and species of gliding mammals, opossums and mice.
Most of the 700 to 900 species of eucalyptus trees are found in Australia. The brightly colored flowers of this classic Australian tree evolved to be pollinated by bats, birds and opossums. The blossoms have large, showy stamens, the male part of the flower; they have no petals. Their long, narrow leaves are toxic except to koalas and a few other leaf eaters. Many species rely on brush fires to open their fruits to release seeds. They contain flammable oils that burst in brush fires, which aids in their propagation.
Seventy percent of Australia is desert or semi-desert. Droughts can last for years and temperatures can rise to 120 degrees Fahrenheit. The seeds of some plants won't germinate until there is rain, then they quickly sprout, grow, reproduce and die in a matter of weeks, leaving behind seeds that wait for the next rain. Other plants grow roots that go deep or spread wide waiting for rain; when it does rain, they gather and store the maximum amount of water. Succulents store water in their leaves, stems and roots. Since plants lose water through their leaves, many desert plants in Australia have leaves that are spiny and small or long and slender with a glossy cover to reflect the heat. Some leaves have a waxy cover to prevent the loss of moisture. To conserve water, some plants have leaves with pores that open only at night. Other plants don't have leaves or drop the leaves they do have when it gets too hot.
Australia's tropical rainforests are located along the northern coasts of Queensland, the Northern Territory and Western Australia. Tropical trees, some of which grow to be 190 feet tall, form a high canopy as they compete for sunlight. Buttress roots have evolved on some trees to give them added support in the moist soil. The tall trees in tropical rainforests shut out the sunlight, so numerous vines have evolved to climb them in search of light. Climbing plants include ferns and epiphytes, plants that have roots that obtain nutrients from the air. Ferns in Australia's tropical rainforests do not have seeds; they reproduce from spores on the bottoms of their leaves.