Steps Toward the Natural System of the Dicotyledons

ABSTRACf The value of vegetative anatomy in phylogenetic analysis is documented. Examples of the use of vegetative anatomy at different taxonomic levels show the continuing role of the anatomical method in building a more natural system of classification of the dicotyledons. The importance of correlating wood and leaf anatomical features with ecological and floristic preferences of taxa is emphasized. Caution is required in basing phylogenetic interpretations upon similarities and differences in xylem structure. The transition from scalariform to simple perforation plates is the only aspect of vessel element evolution that is not potentially reversible and all phylogenetic analyses must reflect this important principle. Some groups outside the Magnoliidae (Annoniflorae) possess very primitive xylems and can be used as evidence to support a pattern of multiple early radiations of the dicotyledons, as well as the increased awareness of a lack of clear subclass definition. The application of anatomical characters in cladistic analysis is reviewed, and the need to utilize strictly homologous features, to provide accurate scoring of character states, and to give proper interpretations to character state transformations is emphasized.