Comparative Anatomy and Morphology of Psiloxylon and Heteropyxis, and the Subfamilial and Tribal Classification of Myrtaceae

Evidence from wood, leaf, and floral anatomy, from palynology, and from reconsideration of vegetative and reproductive morphology fully support (1) the placement of Heteropyxis by Fernandes (1971), Stern & Brizicky (1958), and Weberling (1963) in Myrtaceae/Leptospermoideae on the basis of organography, palynology, and petiole and wood anatomy, and (2) the placement of Psiloxylon by van Tieghem (1904) in Myrtaceae on the basis of especially vegetative anatomy. Diagnostic myrtaceous characters shared by both genera include: entire, simple, gland-dotted, penninerved leaves wth intramarginal veins; rudimentary stipules (Heteropyxis only); pentamery; nectary lining floral tube; imbricate, persistent calyx; many pollen features; sunken styles; baccate or capsular fruits (resp. Psiloxylon and Heteropyxis); exalbuminous seeds; bicollateral bundles; secretory cavities in all aerial organs, including the anthers; unilacunar one-trace nodes; arcuate petiolar bundles with sclerenchymatous sheaths (only in lamina of Psiloxylon); vestured pits; wood fibers with distinctly bordered pits; and tannin. Both genera, however, differ from many, but not all Myrtaceae in having: alternate leaves; unisexual, but bisexual-appearing flowers (only the staminate in Psiloxylon); perigyny; few stamens (Psiloxylon diplostemonous, Heteropyxis essentially obdiplostemonous); short style and divided stigma (Psiloxylon only); cancellate seed (Psiloxylon only); and such wood characters as septate, crystalliferous fibers and lack of vasicentric tracheids in Psiloxylon, and lack of axial parenchyma in Heteropyxis. The segregates Heteropyxidaceae and Psiloxylaceae should thus be included in Myrtaceae. The foregoing ensemble of characters conclusively excludes for Heteropyxis any assignment to Rhamnaceae or Rutaceae, for Psiloxylon any assignment to Bixaceae sensu lato, Flacourtiaceae, or Guttiferae, and for both genera an assignment to Lythraceae. Because it would be anomalous in either of the two traditional myrtaceous subfamilies, Psiloxylon is best treated as a new subfamily in Myrtaceae. Finally, since Leptospermeae and Chamaelaucieae differ as much from each other as either does from Myrteae, Chamaelaucieae should be elevated to subfamilial status in Myrtaceae. The family Myrtaceae thus has four subfamilies: Myrtoideae, Psiloxyloideae, Leptospermoideae, and Chamaelaucioideae. An addendum discusses Briggs & Johnson's (1979) detailed paper on vegetative and especially reproductive structure of Myrtaceae.