Wood, Bark, and Stem Anatomy of Gnetales: A Summary

Abstract

Data from a series of eight papers, representing a survey of Gnetales at the species level, are summarized in order to develop concepts on the relationship between the anatomical data and ecology, phylogeny, and systematics. Most of the characters and character states newly reported have phylogenetic and ecological correlations for Gnetales as a whole rather than systematic significance within the genera. Wood features are sensitively related to habit, although strategies in lianoid species of Ephedra differ from those in lianoid Gnetum species. Data also bear close relationships to organography. Vessel details are given special attention because workers have questioned whether vessels originated in Gnetales independently of those in angiosperms, and thus whether or not vessel origin in the two groups is a synapomorphy. The evidence cited here favors origin of vessels in Gnetales independent of vessel origins in angiosperms Hitherto unappreciated contrasts between vessels of Gnetales and those of angiosperms are detailed: the torusmargo structure of pits in Gnetales (both Ephedra and Gnetum), and the aspiration ability of gnetalean pits represent modes of conduction and promotion of conductive safety different from those in angiosperms, and the shape difference (circular vs scalariform), cited in data matrices of cladograms, is secondary to the different functional syndromes. The intercalation of circular bordered pits into helical secondary wall thickenings of primary xylem tracheary elements of Ephedra and Gnetum (a feature found also in conifers and Ginkgo but not in angiosperms) is given new functional interpretations. Ephedra and Gnetum contain both tracheids and fiber-tracheids that co-occur in a mode different from that in angiosperms which contain both cell types together. Ray structure like that of Gnetales occurs widely within seed plants and likely offers little conclusive information about relationships. Wood data on Gnetales are compatible with the hypothesis that a vesselless group of gymnosperms, such as Pentoxylales or Bennettitales, is the closest sister group of angiosperms. Formation of successive cambia in Gnetum differs from that in Welwitschia: the latter has phellem but no other bark. Because Welwitschia has only phellem, there are more numerous bark features in common between Ephedra and Gnetum than one might have expected. The three genera appear monophyletic on the basis of wood and bark. Infrageneric anatomical features of systematic importance are few, although the arboreal Gnetum gnemon differs from the lianoid Gnetum species in significant ways