A comprehensive introduction to vascular plant phylogeny, the Fourth Edition of Plant Systematics reflects changes in the circumscription of many orders and families to represent monophyletic groups, following the most recent classification of the Angiosperm Phylogeny Group. The taxonomic evidence described includes data from morphology, anatomy, embryology, chromosomes, palynology, secondary plant compounds, proteins, and DNA. Molecular taxonomic methods are fully presented, as are the results of many recent studies, both molecular and morphological. A chapter on the history of plant classification puts current systematic methods into historical context. Issues relating to variation in plant populations and species, including speciation and species concepts, polyploidy, hybridization, breeding systems, and introgression are carefully considered. Appendices cover botanical nomenclature as well as field and herbarium methodology. The text is copiously illustrated, using in large part the informative analytical drawings developed as part of the Generic Flora of the Southeastern United States project.