Leaves, which have fallen into the mud at the bottom of a lake, may leave behind carbon films and impressions that are preserved as the mud slowly turns into rock. A piece of wood, or even a whole tree may be quickly buried in sediment-the absence of oxygen stopping decay from occurring. Ground water then fills all the pores (or little spaces) in the wood with minerals, usually silica. The wood thereby becomes petrified, or turned to stone.A plant fossil is any preserved part of a plant that has long since died.
Kinds of plant fossils
One of the most common kinds of plant fossils is a compression fossil, in which a leaf or flattened part of the plant has been pressed between layers of sediment. Also common are fossil pollen and spores from ancient lake beds, as well as charcoal. Less common, but economically more important, is coal from the plants of Carboniferous swamps. One of the most spectacular of plant fossils is petrified wood.
Coal is formed in a complex process from the partly decayed and compressed remains of ferns and other plants that grew in swampy conditions. Brown coal is generally of more recent origin than black coal and contains more water. Victoria's Latrobe Valley has large deposits of brown coal (less than 65 million years old), while black coal is found particularly in New South Wales and Queensland (200 to 300 million years old).Plants have evolved over time. Australia's fossil record shows that during the Jurassic Period the dominant plants were Cycads, gingkoes, conifers and ferns. In the next large time period, the Cretaceous Period, flowering plants made their first appearance. One of the oldest known flowers was discovered in 115-118 million year old rocks at Koonwarra (south Gippsland) in 1989.
Cycads are sometimes called 'living fossils' because they reached their peak in the Jurassic Period (141 to 205 million years ago). Cycads are a member of the order Cycadales, which are the most primitive seed bearing plants. Like the related conifers (order Coniferales), Cycads are cone bearing. They are a group of seed plants characterized by a large crown of compound leaves and a stout trunk. They are evergreen, gymnospermous, dioecious plants having large pinnately compound leaves.
These ancient conifers reached their peak in the Mesozoic (251 to 65 million years ago). They are named after a group of South American Indians, the Araucani. The common names of the araucaria include the Hoop Pine, Bunya Bunya Pine, Norfolk Island Pine, Kauri Pine and the South American Monkey Puzzle Tree.
Ferns are one of the most ancient plant groups surviving today. They first appear in the fossil record in the Carboniferous Period, over 300 million years ago. Ferns are now confined to fairly wet areas but once dominated the Earth's forests.
Gingkoes have been found as 240 million year old fossils. The formerly extensive family of gingkoes has been reduced to just one species, Gingko biloba. The discovery of this species in the gardens of some Chinese monasteries a century ago surprised scientists, who had thought it extinct. Gingkoes are gymnosperms, and are therefore related to conifers.