Wood Anatomy of Gnetales in a Functional, Ecological, and Evolutionary Context

Abstract

New scanning electron microscope (SEM) and light microscope data and illustrations are presented in order to compare hydraulic adaptations of non-gnetalean conifers and angiosperms to relevant wood features of Gnetales. Gnetales have essentially all of the adaptations of both groups, yet have not competed well, despite predating angiosperms in origin and radiation. Angiosperms may be advantaged more by life cycle abbreviation and by heterochronic possibilities than by wood features. Wood features of Gnetales that relate to conduction (torus-margo differentiation of tracheid pit membranes, distribution of tori within the wood, perforation plate simplification, growth rings, vesturing, helical thickenings, and axial parenchyma) are reviewed in the light of recent work that demonstrates the physiological significance of these features in angiosperms. The various xylary adaptations of Ephedra, Gnetum, and Welwitschia are analyzed in terms of conductive efficiency versus conductive safety, and in turn, compared to the habitats of the three genera, respectively: Ephedra and Welwitschia survive in rather extreme habitats, whereas Gnetum competes in mesic forest by what may be minor shifts among basically similar niches. Gnetales have essentially all of the wood anatomical advantages of angiosperms, except for ability to shift degrees of wood anatomical juvenilism or adulthood—an important angiosperm advantage. The relatively lengthy life cycle of Gnetales, requiring female gametophyte formation prior to embryo formation (vs. simultaneous endosperm and embryo development in angiosperms) and the related requirement for at least several years’ growth prior to seed formation are probably the prime reasons for lack of gnetalean success. Wood features should be interpreted as adapted to the present-day ecology of a woody species, and ideas that evolutionary flexibility to shift into more seasonal conditions is hindered by rigid wood formulas (which actually can change rather rapidly) should be questioned. Wood evolution of Gnetales, like that of angiosperms, can be regarded as a series of shifts in the trade-offs between conductive efficiency and conductive safety.