One of the objectives of systematists is to discover a phylogenetic system of classification. The evolutionary history of gymnosperms extends far back in geological time (Figs. 1.1, 1.2); its fossil record begins in the Upper Devonian and spreads over nearly 350 my B.P. (million years Before Present) (Beck 1985). Most branches of the phylogenetic tree are extinct or have left only fragmentary palaeobotanical records. Several presumed gymnospermous remains have been described from the Upper Devonian and Lower Carboniferous. It is, therefore, natural that there will be diversity of opinion on the phylogenetic relationships and classification of plants. However, it is felt that classification schemes should be flexible, and should be continuously revised in the light of growing knowledge (Arnold 1948). The evolutionary relationships of fossil plants are, in most instances, not completely known (Miller 1985). Until reproductive organs are discovered in organic connection with the vegetative parts, their taxonomic. placement is entirely optional (Sporne 1965).
KeywordsFossil Record Secondary Xylem Fossil Plant Leaf Trace Phylogenetic System
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