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Preface to "Linear sequence, classification, synonymy, and bibliography of vascular plants: Lycophytes, ferns, gymnosperms and angiosperms"

Organizing knowledge of the natural world into a system aids communication and helps us to understand better the organisms that surround us and upon which we depend. These classifications are often embedded in cultural and social systems, and in the past such “folk taxonomies” have been absorbed into scientific taxonomy. The basic function of words is that of naming (Lyons 1977), and in the human mind nothing really exists or can be communicated without a name. Anything can be classified, but modern biological classification is different because it involves evolutionary relationships. Similarity between organisms placed in the same taxon is not arbitrary but is a result of shared descent from their common ancestor. Before the age of molecular phylogenetics, placement of organisms was based on morphological homology, but this can sometimes be deceptive (for example, Nelumbonaceae and Nymphaeaceae are aquatics and have superficially similar leaves, flowers and other features; they have previously been thought to be closely related, but molecular and detailed morphological data show they are not closely related at all). Linear sequences of plant families are required by herbarium curators who wish to arrange collections systematically rather than alphabetically, and several herbaria are currently in the process of reorganising their collections, which requires linear classifications. Currently there is a wide range of systems in use, but as the understanding of relationships has changed, many of these classifications have become outdated and no longer represent the best estimate of relationships. To organise flowering plants systematically in floras, books, indices, check-lists, order beds (e.g. in botanical gardens) and listings on the internet, linear sequences based on resolved consensus classifications have also proven to be useful (Haston et al. 2007, 2009). When a linear sequence is derived from branching phylogenetic trees, the order of taxa is dependent on the methodology used, and there is an inevitable loss of phylogenetic information. The linear sequences presented in this volume were derived from phylogenetic trees using the methodology described by Haston et al. (2007; even though this method was questioned by Hawthorne et al. 2008). In this method, clades with few taxa are placed before those with many taxa, which is of course an arbitrary choice. This volume includes three articles, the first concerning lycophytes and ferns (Christenhusz et al. 2011a), the second providing a system for gymnosperms (Christenhusz et al. 2011b) and the third dealing with synonymy and bibliography of angiosperms (Reveal & Chase 2011). Lycophytes and ferns. The classification of ‘pteridophytes’ has long been in flux. Species have been placed in a number of genera, moved from one family to another, and this has led to many taxonomists losing track of the currently accepted position of fern species. Recent classifications based on molecular data were a