Forest Vines to Snow Tussocks: The Story of New Zealand Plants
These are principally the spores of ferns, bryophytes and other groups and the pollen grains (Fig. 122) of conifers and flowering plants. They are useful in interpreting past floras because of their great resistance to decay (with some exceptions), and because of a number of distinctive and constant characteristics which often enable them to be identified to family, genus or, less often, species. The resistance to decay derives page 239from a wax-like compound in the cell walls. The features useful in identification are the shape of the grain (spherical, triangular, bean-like etc); the number, form and arrangement of germination pores; the structure of the wall; and the sometimes intricate pattern of sculpturing on the surface of the spore or grain.
Some plants are more likely to be fossilised than others. Lowland plants are better represented than alpine plants because it is in the lowlands that deposition is prevalent. The fossilisation process may begin in suitable alpine sites such as bogs and ponds, but it is likely to be interrupted by erosion at an early stage. Firm or even hard textured plant parts make better fossils than delicate parts that tend to shred or decay rapidly. Plant parts which separate readily, such as leaves with abscission layers, are more common as fossils than parts which normally remain attached and slowly decay. Spores and pollen which are wind dispersed are produced in great quantities and spread for great distances, so are often over-represented in the fossil record by comparison with bird, water or insect-pollinated species.