Wood anatomy of Mackinlaya and Apiopetalum (Araliaceae) and its systematic implications

Wood anatomy was examined in both species of Apiopetalum (trees endemic to New Caledonia) and in two species of Mackinlaya (shrubs from Queensland, Australia), using light and scanning electron microscopy, to investigate their phylogenetic relationships and taxonomic position within Apiales. These genera share several wood features: small intervessel pits, ranging from 3 to 6 μm; both paratracheal and apotracheal (diffuse and diffuse-in-aggregates in Apiopetalum only) axial parenchyma; non-septate fibers; rays composed of mostly upright and square cells; and brown deposits in the vessels. Differences in wood structure between the genera appear to be related to habit. The results confirm recent molecular sequence data suggesting that Apiopetalum and Mackinlaya form a monophyletic group. Evidence from wood anatomy neither validates nor refutes the hypothesis that these two genera are intermediate between Araliaceae and Apiaceae and offers no clear indication of the group's phylogenetic position. Several wood characters (small intervessel pits, thick fiber walls, non-septate fibers) suggest a relationship with Myodocarpus, Delarbrea, and Pseudosciadium. No clear synapomorphies were found to support a sister relationship between Apiopetalum and Mackinlaya and core Apiaceae as previously suggested by Plunkett, nor between them and other Araliaceae. Helical thickenings on the walls of both ray and axial parenchyma, previously reported only once in Trigonia (Trigoniaceae), were observed in one sample of M. macrosciadea.