COMPARATIVE ANATOMY AND MORPHOLOGY OFPS1LOXYLONANDHETEROPYXIS, AND THE SUBFAMILIAL AND TRIBAL CLASSIFICATION OF MYRTACEAE

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Abstract
Summary: Evidence from wood, leaf, and floral anatomy, from palynology, and from reconsideration of vegetative and reproductive morphology fully support (1) the placement ofHeteropyxisby 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 ofPsiloxylonby 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 (Heteropyxisonly); pentamery; nectary lining floral tube; imbricate, persistent calyx; many pollen features; sunken styles; baccate or capsular fruits (resp.PsiloxylonandHeteropyxis); 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 ofPsiloxylon); 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 inPsiloxylon); perigyny; few stamens (Psiloxylondiplostemonous,Heteropyxisessentially obdiplostemonous); short style and divided stigma (Psiloxylononly); cancellate seed (Psiloxylononly); and such wood characters as septate, crystalliferous fibers and lack of vasicentric tracheids inPsiloxylon, and lack of axial parenchyma inHeteropyxis.The segregates Heteropyxidaceae and Psiloxylaceae should thus be included in Myrtaceae. The foregoing ensemble of characters conclusively excludes forHeteropyxisany assignment to Rhamnaceae or Rutaceae, forPsiloxylonany assignment to Bixaceaesensu 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,Psiloxylonis 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.

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