Forest Vines to Snow Tussocks: The Story of New Zealand Plants
First of all it is usually detached bits of plants rather than whole plants that become fossils, and this of course makes identification more difficult. Of macrofossils (visible to the naked eye) the most common are leaves, which often detach cleanly from stems along a special layer of weak cells, then twigs, and, less commonly, cones of conifers and fruits and seeds of flowering plants. Unfortunately flowers, which are the most reliable means of identification, are mostly soft tissued and often decay before they can become fossils.
Figure 121 (left) Fossil leaf compression about 7x4 cm, of the extinct Nothofagus oliveri, from Nuggety creek, near Murchison. Mid-Miocene age. Photo: J. E. Casey.
Figure 122 (above) Pollen of zygoynum baillonii of New Caledonia. Zygogynum belongs to the primitive family Winteraceae and at the present day is restricted to New Caledonia. The distincative fossil pollen of this or a related genus has been found in New Zelands, in a fossil assemblage of Middle Pliocene Age. Photo: F. B. Sampson.
The slow but steady accumulation of organic material in swamps and bogs can also lead to the formation of fossils as the conditions at such sites are often acidic and inhibit decay organisms. The plant remains, which form peat below the living plants at the surface, are termed subfossils. If peat becomes deeply buried, particularly under layers of inorganic sediments, pressure and heat gradually convert it into coal and the plant parts steadily lose their structure until none remains in the highest grade coals.
Plants may also be fossilised in fine textured volcanic ash redistributed by the heavy rain often associated with eruptions.
Sometimes the complete cellular structure of fossils is preserved. This happens when plant material, most often in swamps, becomes impregnated and replaced by silicates (sometimes near silica springs), carbonates and similar compounds in solution. After such an event the replaced plant material becomes resistant to physical and chemical change and the preservation of cells and sometimes cell contents can be remarkably good even in fossils hundreds of millions of years old. By special techniques thin sections can be made of these well preserved fossils and cell structure can be studied in detail.