Wood anatomy of Mackinlaya and Apiopetalum (Araliaceae) and its systematicimplications

Wood anatomy was examined in both species of Apiopetalum (trees endemic to New Caledonia) and in two species of Mackinlaya (shrubs from Queensland, Au stralia), using light and scanning electron microscopy, to investigate thei r phylogenetic relationships and taxonomic position within Alpiales. These genera share several wood features: small intervessel pits, ranging from 3 to 6 mu m; both paratracheal and apotracheal (diffuse and diffuse-in-aggreg ates in Apiopetalum only) axial parenchyma; non-septate fibers; rays compos ed of mostly upright and square cells; and brown deposits in the vessels. D ifferences in wood structure between the genera appear to be related to hab it. The results confirm recent molecular sequence data suggesting that Apio petalum and Mackinlaya form a monophyletic group. Evidence from wood anatom y neither validates nor refutes the hypothesis that these two genera are in termediate between Araliaceae and Apiaceae and offers no clear indication o f the group's phylogenetic position. Several wood characters (small interve ssel pits, thick fiber walls, non-septate fibers) suggest a relationship wi th 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 p arenchyma, previously reported only once in Trigonia (Trigoniaceae), were o bserved in one sample of M. macrosciadea.