Wood Anatomy of Argophyllaceae (Asterales): Adaptation in a Small Clade

Abstract

Argophyllaceae (Argophyllum, 14 spp.; Corokia, 6 spp.; Lautea, 1 sp.), are shrubs that occur in the southwestern Pacific and eastern Australia. They occur in habitats where moisture is relatively common but dry days and mild frost may occur. The woods of these genera show enough distinctive features to justify their grouping in a single family: perforation plates with 10–20 bars, vessel elements narrow and numerous per mm2, imperforate tracheary elements about 50% longer than the vessel elements, axial parenchyma scarce, diffuse, multiseriate rays narrow and heterocellular (upright cells common in uniseriate rays), crystals absent, gum deposits common. These features group the genera of Argophyllaceae more closely with each other than with the nearest families in Asterales (Alseuosmiaceae, Phellinaceae). Probable apomorphies of the genera include helical thickenings in vessels and tracheids, together with abundant tracheids and rare septate fiber-tracheids (Corokia); almost total absence of axial parenchyma and tracheids combined with maximal abundance of septate fiber-tracheids and no helical thickenings (Argophyllum, Lautea). Lautea, formerly included within Corokia, has floral and foliar distinctions and is endemic to a single island, Rapa Iti. Woods of Argophyllaceae are alike in their ecological adaptations (perforation plates, vessel diameter and density) but the presence of tracheids and helical thickenings in Corokia suggest adaptations to frost and mild drought. As expected, vessels group more prominently in the tracheid-free species (Argophyllum, Lautea) but very little in the tracheid-rich genus Corokia.