Petalostigma F.Muell.
Hooker's J.Bot.Kew Gard.Misc. 9:16 (1857)

Name Status: Current
Browse to the list of specimens for Petalostigma F.Muell.

Scientific Description
H.R. Coleman, Thursday 8 September 2016

Family Euphorbiaceae.

Habit and leaf form. Trees, or shrubs; evergreen; non-laticiferous and without coloured juice. Plants non-succulent; unarmed. Mesophytic, or xerophytic. Leaves small to medium-sized; alternate; spiral; leathery; petiolate. Petioles wingless. Leaves non-sheathing; gland-dotted, or not gland-dotted; simple. Leaf blades entire; pinnately veined. Leaves with stipules (inconspicuously). Stipules scaly; caducous. Leaf blade margins entire, or crenate. Leaves without a persistent basal meristem. Leaf anatomy. Hairs present; glandular hairs absent; complex hairs absent. Branched hairs absent. Urticating hairs absent. Extra-floral nectaries absent. Stem anatomy. Nodes tri-lacunar, or unilacunar. Secondary thickening developing from a conventional cambial ring, or anomalous; from a single cambial ring.

Reproductive type, pollination. Fertile flowers functionally male, or functionally female. Unisexual flowers present. Plants dioecious. Female flowers aggregated in ‘inflorescences’; without staminodes. Male flowers aggregated in ‘inflorescences’; without pistillodes, or with pistillodes (obscure, or exceptionally developed as an irregularly bilobed structure). Entomophilous.

Inflorescence and flower features. Flowers aggregated in ‘inflorescences’. Inflorescence few-flowered. Flowers when aggregated, in racemes, or in fascicles. Inflorescences simple; axillary. Flowers pedicellate, or sessile (sometimes in females); bracteate; minute to small; regular. Floral receptacle developing an androphore (androphore shortly conical), or with neither androphore nor gynophore (? depending on interpretation). Free hypanthium absent. Hypogynous disk absent. Perianth sepaline; 4–8; 1 -whorled, or 2 -whorled (the outer whorl, when present, longer and narrower than the inner whorl); isomerous, or anisomerous. Calyx present; 4–8; 1 -whorled, or 2 -whorled; polysepalous; erect, or spreading; at least sometimes hairy; imbricate; non-fleshy; not persistent (on female flowers). Corolla absent. Androecium present, or absent. Fertile stamens present, or absent (female flowers). Androecial members indefinite in number. Androecium 18–86. Androecial members branched; free of the perianth; coherent (shortly connate into a column). Androecium exclusively of fertile stamens. Stamens 18–86; all more or less similar in shape; polystemonous; erect in bud, or inflexed in bud. Filaments not geniculate. Anthers all alike; dorsifixed; dehiscing via longitudinal slits; extrorse; bisporangiate, or tetrasporangiate. Fertile gynoecium present, or absent (male flowers). Gynoecium 3 carpelled, or 4 carpelled. The pistil 3 celled, or 4 celled. Carpels isomerous with the perianth. Gynoecium syncarpous; synovarious, or synstylovarious; superior. Ovary plurilocular; 3 locular, or 4 locular. Gynoecium stylate. Styles 3, or 4; free (each expanded, petal-like and sometimes lobed towards the apex); simple; apical; becoming exserted; deciduous; hairless. Stigmas 3, or 4; expanded, petal-like and sometimes lobed towards the apex; dry type; papillate, or non-papillate; Group II type. Placentation axile, or apical. Ovulodes absent. Ovules 2 per locule; pendulous; epitropous; with ventral raphe, or with dorsal raphe; collateral; arillate; anatropous.

Fruit and seed features. Fruit fleshy; yellow, or yellow to red; hairy; dehiscent, or a schizocarp (with a succulent exocarp and a hard endocarp). Mericarps 3, or 4. Fruit when dehiscent, a capsule (globular to ovoid). Capsules septicidal. Fruit 3 celled, or 4 celled; 3 locular, or 4 locular; elastically dehiscent (schizocarpic capsules often splitting elastically), or passively dehiscent. Seeds 1 per locule, or 2 per locule; 2 or more per mericarp. Seeds ovoid to ellipsoid, slightly compressed; endospermic. Endosperm oily. Seeds arillate. Cotyledons 2 (usually wider than the radicle). Embryo straight, or curved. Testa hard; smooth; homogeneous in colour; brown, or red. Seedling. Germination phanerocotylar, or cryptocotylar.

Physiology, biochemistry. Mustard-oils present, or absent.

Geography, cytology, number of species. World distribution: Australia and New Guinea. Native of Australia. Not endemic to Australia. Australian states and territories: Western Australia, Northern Territory, Queensland, and New South Wales. Northern Botanical Province. A genus of 6 species.

Additional characters Perianth of male flowers sepaline; 4. Perianth of female flowers sepaline; 6–8.

Taxonomic Literature

  • Wheeler, J. R.; Rye, B. L.; Koch, B. L.; Wilson, A. J. G.; Western Australian Herbarium (1992). Flora of the Kimberley region. Western Australian Herbarium. Como, W.A.