Flora of North America species comparison

Species ca. 80 (12 in the flora).

There has been controversy surrounding the taxonomic status of Argythamnia. Some authors have recognized Ditaxis, which includes all of the species in the flora area, at the generic level (G. L. Webster 1994b; A. Radcliffe-Smith 2001), whereas others have treated it as a subgenus of Argythamnia (J. W. Ingram 1980; R. McVaugh 1995). There are several morphological characters that distinguish these taxa and pollen morphology supports their generic recognition (W. Punt 1962). However, recent molecular phylogenetic studies demonstrate that recognizing Ditaxis makes Argythamnia paraphyletic (Y. Ramírez-Amezcua 2011), so they are treated here as a single genus.

Argythamnia heterantha (Zuccarini) Müller Arg., from Mexico, is cultivated; the seeds are used as a saffron substitute and represent a potential resource for dye, oil, and protein (M. D. Méndez-Robles et al. 2004).

M. C. Johnston (1990) reported Argythamnia astroplethos J. W. Ingram from the Chinati Mountains, Presidio County, Texas, but no specimens were cited and none have been located. This species grows nearby in Chihuahua, Mexico, and may eventually be documented from Texas. It belongs to subgenus Chiropetalum (A. Jussieu) J. W. Ingram and can be distinguished from other Argythamnia species in the flora area by its indumentum of stellate hairs in addition to malpighiaceous hairs, tetramerous staminate flowers, and styles that are twice 2-fid.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Genera ca. 220, species ca. 6500 (24 genera, 259 species in the flora).

Molecular phylogenetic studies have shown Euphorbiaceae, as traditionally treated (A. Radcliffe-Smith 2001; G. L. Webster 1994b, 2014), to be polyphyletic, forming several groups in Malpighiales (C. C. Davis et al. 2005; T. Tokuoka and H. Tobe 2006; K. Wurdack and Davis 2009; Z. Xi et al. 2012). The treatment here reflects those findings. Genera formerly placed in subfamilies Oldfieldioideae Eg. Köhler & G. L. Webster and Phyllanthoideae Beilschmied, characterized by two ovules per locule, are treated as Picrodendraceae and Phyllanthaceae respectively, with two genera segregated from the latter as Putranjivaceae; the first two families appear to be sister taxa, possibly near much of the remaining traditional Euphorbiaceae, whereas the third is placed elsewhere in Malpighiales (Xi et al. 2012). The remaining genera, characterized by one ovule per locule, are treated as Euphorbiaceae in the strict sense and Peraceae Klotzsch (not represented in the flora area).

Four subfamilies currently are recognized in the narrowly defined Euphorbiaceae (K. Wurdack et al. 2005; G. L. Webster 2014). Traditionally, three subfamilies, Acalyphoideae Beilschmied, Crotonoideae Beilschmied, and Euphorbioideae Beilschmied, were recognized based on laticifer presence or absence and pollen morphology (A. Radcliffe-Smith 2001; Webster 1994b). Phylogenetic analyses of molecular data (Wurdack et al.; T. Tokuoka 2007) support the monophyly of Euphorbioideae, represented in the flora area by Ditrysinia, Euphorbia, Gymnanthes, Hippomane, Hura, Microstachys, Pleradenophora, Stillingia, and Triadica. Most of the traditional Crotonoideae is moderately supported as monophyletic, including Astraea, Cnidoscolus, Croton, Jatropha, Manihot, and Vernicia in the flora area. These studies also strongly support the monophyly of most of Acalyphoideae (including Acalypha, Adelia, Argythamnia, Bernardia, Caperonia, Dalechampia, Mercurialis, Ricinus, and Tragia in the flora area), and segregation of a fourth, small subfamily, Cheilosoideae K. Wurdack & Petra Hoffmann (not represented in the flora area). The remaining genera of Acalyphoideae and Crotonoideae, none of which are in the flora area, were placed in several small clades whose relationships to each other and to the four subfamilies were not well supported.

Euphorbiaceae in the strict sense are diverse morphologically, but most are characterized by schizocarpic capsules in which the cocci separate from the persistent columella, often explosively; the seeds are dispersed when the cocci split septicidally and usually also loculicidally. Similar capsules also are found in many Phyllanthaceae and Picrodendraceae but, as noted above, they have two ovules per locule, versus one in Euphorbiaceae. White or whitish latex is found in Euphorbioideae and colored latex in many Crotonoideae, whereas Acalyphoideae lack latex. Pseudanthia evolved independently and are structurally different in Dalechampia and Euphorbia.

Euphorbiaceae are most species-rich and ecologically important in tropical and subtropical regions. The same is true of genera found in the flora area, many of which are represented there only by a small proportion of their global diversity.

Notable economically important Euphorbiaceae are Hevea Aublet, a major source of rubber; Manihot, cassava or manioc, the starchy tubers of which are a major food source in much of the tropics; Ricinus, the source of castor oil; and Vernicia (and its close relative Aleurites J. R. Forster & G. Forster), sources of tung and other finishing oils. Some species of Euphorbiaceae are used horticulturally, especially members of Euphorbia; probably the best known of these is the poinsettia, E. pulcherrima Willdenow ex Klotzsch. Also well-known is Codiaeum variegatum (Linnaeus) A. Jussieu, the horticultural croton. Some introduced Euphorbiaceae have become problematic invasives in the flora area, particularly in areas with mild climates. Most notable among these invasive species are leafy spurge (Euphorbia virgata, commonly incorrectly called E. esula), Chinese tallowtree (Triadica sebifera), and tung-oil tree (Vernicia fordii).

Mallotus japonicus (Linnaeus f.) Müller Arg., food wrapper plant, has escaped locally in Durham and Orange counties, North Carolina. The Orange County population may have been eradicated and the status of the Durham County population is not known. This dioecious shrub or small tree, native to eastern Asia, has stellate hairs and in the key below would come under the first lead of couplet 5 with Astraea, Bernardia, and Croton. It can be distinguished from those three genera by its pale yellow to white glandular scales on the leaves and stems, staminate flowers with 70–100 stamens, unbranched styles, and ovaries and capsules covered with soft spines and reddish orange glandular scales.

Sapium haematospermum Müller Arg. from South America was collected on ballast in Pensacola, Florida, in 1901; this collection generally has been incorrectly reported as S. glandulosum (Linnaeus) Morong. Although the species does not appear to have become naturalized in the flora area, it could become adventive in subtropical areas. In the key below, S. haematospermum would come under the second lead of couplet 12 and can be distinguished from the five genera there by its combination of petioles with apical glands, inflorescences with two glands subtending each bract, staminate flowers with two to three sepals that are connate basally, pistillate flowers with three-carpellate pistils bearing three styles, and seeds covered in a red aril.

In the key and descriptions below, laminar glands are those borne on the surface of the leaf blade, not those extending from the margins or teeth or borne at the leaf base at the junction with the petiole.

(Discussion copyrighted by Flora of North America; reprinted with permission.)