Parallelism and Reversibility in Xylem Evolution a Review

Abstract

The irreversibility of the major trends of xylem evolution, such as the origin of vessels in primitive angiosperms with long fusiform initials, and the shifts from scalariform to simple perforations and from tracheids 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 and perforation plate type. Some recent phylogenetic analyses of seed plants suggest that there also have been some reversals in these general trends. The likelihood and extent of parallel origins and reversions of the major trends in xylem specialization are explored here by analysing a number of published hypotheses on the phylogenetic relationships within wood anatomically diverse major clades of angiosperms, and within some individual families. On the basis of these analyses, it appears that for these major Baileyan transformation series, parallelisms were more than twice as common as reversals. Functional adaptations to increased efficiency and safety of hydraulic architecture can largely explain the high incidence of parallelisms in xylem evolution.