PARALLELISM AND REVERSIBILITY IN XYLEM EVOLUTION - A REVIEW

The irreversibility of the major trends of xylem evolution, such as th e origin of vessels in primitive angiosperms with long fusiform initia ls, and the shifts from scalariform to simple perforations and from tr acheids to libriform fibres, has long been accepted by wood anatomists . Parallel development of these and other xylem features is generally accepted, and is suggested by the distribution patterns of the fibre a nd perforation plate type. Some recent phylogenetic analyses of seed p lants suggest that there also have been some reversals in these genera l trends. The likelihood and extent of parallel origins and reversions of the major trends in xylem specialization are explored here by anal ysing a number of published hypotheses on the phylogenetic relationshi ps within wood anatomically diverse major clades of angiosperms, and w ithin some individual families. On the basis of these analyses, it app ears that for these major Baileyan transformation series, parallelisms were more than twice as common as reversals. Functional adaptations t o increased efficiency and safety of hydraulic architecture can largel y explain the high incidence of parallelisms in xylem evolution.