The appearance of land vegetation on Earth 450 million years ago marked a period of unparalleled innovation in plant evolution. The transition from algae to the first land plants---the transition from water to air---entailed adaptations that gave rise to many of today's major plant groups, including mosses, liverworts, lycopsids, and ferns. An understanding of early land plant relationships is critical to a full-scale appreciation of phylogenetic patterns in the plant kingdom. The first comprehensive application of cladistics---a system of defining taxa by shared characteristics to infer evolutionary relationships--- to the massive body of data on both living and fossil plants, this book clarifies phylogenetic patterns within and among basal groups of land plants. Summarizing the morphological and molecular evidence available, the authors critically explore the distribution of characters such as stem branching, leaves, and heterospory. Their specific phylogenetic hypotheses make explicit previous morphology-based studies, and their inclusion of fossils clarifies relationships among extinct groups. The book contributes significantly to current ideas on the homology of land plant structural features and supports the monophyly of vascular plants as well as the early divergence of lycopsids from other tracheophytes. Illustrated with line drawings and complete with appendices detailing the morphology of early fossil plants and their living relatives, The Origin and Early Diversification of Land Plants discusses the implications of its phylogenetic conclusions for understanding the evolution of land plant structure, life cycles, the appearance of groups in the fossil record, biogeographic patterns, and related geological events. In its detailed analysis of the patterns and processes underlying the origin of land plants, the book sheds light on central questions surrounding the initial assembly of terrestrial ecosystems.