The occurrence of Scalariform perforation plates and helical vessel wall thickenings in wood of Myrtaceae

Abstract

The occurrence of multiple perforation plates and helical wall thickenings in vessel elements of 144 species (plus 12 varieties and 2 hybrids) in 53 genera of Myrtaceae was extensively explored. Scalariform perforation plates occur in 40 species (plus 1 hybrid), in Luma, Myrceugenia, Myrteola, Ugni, and in the monotypic Myrtastrum rufo-punctatum, Neomyrtus pedunculata, and Tepualia stipularis. Ugni candollei also has foraminate (i.e., sieve-like) perforation plates. Helical wall thickenings occur in 33 species (plus 1 hybrid), in Acmena, Austromyrtus, Myrceugenia, Myrcia, Myrcianthes, Psidium, Xanthomyrtus, and in Myrtus communis. Most of these records are new. The species with exclusively scalariform perforation plates (in Luma, Myrteola, Neomyrtus, and Ugni) are from cool mesic habitats; those with mixed simple and multiple perforation plates are also largely cool mesic but show a somewhat greater diversity of habitats. Myrtaceae with exclusively simple perforation plates predominate in all habitat types. Helical wall thickenings occur sporadically throughout the distributional range of the family. However, tropical species tend to have weaker helical thickenings than the subtropical and temperate species exhibiting them. The possible functional significance of these ecological tendencies is discussed. It is hypothesised that multiple perforation plates were retained in some cool mesic Myrtaceae because of a lack of strong selective pressure to eliminate them from this type of environment, rather than that they were retained because of adaptive significance in trapping embolisms. The systematic and diagnostic value of multiple perforation plates and helical wall thickenings is also discussed. Scalariform plates are largely confined to related genera in Myrtoideae; Tepualia is the only representative from Leptospermoideae. Helical wall thickenings are only of limited diagnostic and systematic value above the species level.