Systematic Anatomy of Euphorbiaceae Subfamily Oldfieldioideae I. Overview

The biovulate subfamily Oldfieldioideae of Euphorbiaceae, characterized by spiny pollen, is an otherwise apparently diverse assemblage of mostly Southern Hemisphere trees and shrubs that traditionally have been allied with genera of Phyllanthoideae and Porantheroideae sensu Pax and Hoffmann. Although fairly diverse anatomically, the following structures characterize the subfamily with only a few exceptions: pinnate brochidodromous venation with generally randomly organized tertiary and higher order venation; foliar and petiolar glands absent; unicellular or unbranched uniseriate trichomes; latex absent; mucilaginous epidermis or hypodermis; brachyparacytic stomata; vessel elements with simple perforation plates and alternate, often very small, intervascular pits; thick-walled nonseptate imperforate tracheary elements; numerous narrow heterocellular rays; and abundant axial xylem parenchyma in diffuse to somewhat banded patterns and often bearing prismatic crystals. Anatomically, the shrubby Australian ericoid genera form a well-defined group with obvious affinities to the more arborescent Australasian genera, which show clear relationships to each other; the African and neotropical genera bearing compound leaves form another distinct group; the remaining genera are somewhat more isolated and seem to represent, in various cases, elements that are primitive within the subfamily or elements derived from the group bearing compound leaves. Presence of theoid teeth and palmately compound leaves in Oldfieldioideae are features consistent with Dilleniid origin for Euphorbiaceae. As a taxonomic entity the euphorbiaceous subfamily Oldfieldioideae K6hler & Webster dates conceptually from the palynological studies of Punt (1962) and K6hler (1965) who noted the spiny pollen that characterizes the group; the assemblage recognized by pollen structure was subsequently formalized nomenclaturally as a new subfamily (Webster, 1967). In essence, pollen characters circumscribed a group of genera that previously had been assigned to the biovulate subfamilies Phyllanthoideae and Porantheroideae in the system of Pax & Hoffmann (1931). Webster's first (1975) classification of oldfieldioid genera contained several novel taxonomic associations at variance with earlier classifications, notably those of Pax & Hoffmann (1931) and Hutchinson (1969). Interest in the anatomy of Oldfieldioideae stemmed from the need for comparative data to aid placement of the problematic genus Picrodendron Planchon (Hayden, 1977). Despite a number of publications dealing with the anatomy of oldfieldioid genera (Appendix 2), detailed information for most of the subfamily is lacking. Once Picrodendron was shown to be a member of Oldfieldioideae (Hayden, 1977; see also Hayden et al., 1984, and Hakki, 1985), an anatomical survey of the entire subfamily was initiated (Hayden, 1980) to assess relationships from data independent of reproductive (including pollen) morphology. Based in part on these anatomical studies, classification of Oldfieldioideae has been modified from Webster's (1975) original proposal; two broadly similar classifications, one by G. L. Webster (the preceding issue) and another by G. A. Levin and M. G. Simpson (this issue), are proposed elsewhere.