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Common name. Milkweed Family.
Family Sometimes included in Apocynaceae.
Habit and leaf form. Herbs, or herbaceous climbers, or lianas, or shrubs, or trees (rarely); laticiferous. ‘Normal’ plants, or switch-plants, or plants of very peculiar form; somtimes ‘cactoid’. Leaves well developed (with ‘pitchers’ in Dischidia), or much reduced. Plants succulent, or non-succulent. Perennial. Self supporting, or climbing; when climbing stem twiners, or scrambling. The twiners twining anticlockwise (Araujia, Ceropegia, Stephanotis). Mesophytic, or xerophytic. Leaves opposite (decussate, usually), or whorled (rarely, and rarely spiral); ‘herbaceous’, or fleshy, or membranous, or modified into spines; simple. Leaf blades entire (often reduced); one-veined, or pinnately veined, or pinnately veined to palmately veined. Leaves usually without stipules. Domatia recorded (in 3 genera); represented by pits, or hair tufts. Stem anatomy. Nodes unilacunar. Secondary thickening developing from a conventional cambial ring, or anomalous; from a single cambial ring.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Entomophilous; often via diptera. Pollination mechanism conspicuously specialized (involving trapping of insects’ legs or probosces between the osmotically elastic anther wings, and withdrawal entailing capture of the pollinia by means of ‘sutured corpuscular pollen carriers’).
Inflorescence and flower features. Flowers solitary, or aggregated in ‘inflorescences’. The terminal inflorescence unit usually cymose (often umbelliform), or racemose (rarely). Flowers fragrant, or malodorous, or odourless; regular; 5 merous; cyclic; tetracyclic. Hypogynous disk absent. Perianth with distinct calyx and corolla; 10; 2 -whorled; isomerous. Calyx 5; 1 -whorled; usually gamosepalous (at the base); imbricate, or valvate; regular; with the median member posterior. Corolla 5; 1 -whorled; appendiculate (with a corona, simple or of separate scales), or not appendiculate; gamopetalous (the tube short); contorted; regular. Androecium 5. Androecial members adnate (to the base of the corolla tube); united with the gynoecium (forming a gynostegium with it); coherent (in a short sheath around the style — by contrast with Periplocaceae); 1 - adelphous; 1 -whorled. Androecium exclusively of fertile stamens. Stamens 5; isomerous with the perianth; oppositisepalous; all alternating with the corolla members; filantherous to with sessile anthers. Filaments appendiculate (the short filaments ornamented from their external bases with the nectariferous components of a corona of variable form, which is incorporated in the gynostegium). Anthers cohering; basifixed; introrse; bilocular, or four locular; bisporangiate (usually), or tetrasporangiate (in Secamone); appendaged (provided with horny wings, contributing to the coronal complex). Pollen shed in aggregates; in the form of pollinia (one or two per theca). Gynoecium 2 carpelled. The pistil 2 celled. Carpels reduced in number relative to the perianth. Gynoecium syncarpous (but the carpels united only by their styleheads); synstylous; superior. Ovary plurilocular; 2 locular (insofar as the separate, unilocular ovaries can be regarded as the ‘locules’ of a ‘syncarpous’ gynoecium). Gynoecium median; stylate. Styles 2; partially joined (free below, but united by the dilated stylehead, which has lateral stigmatic surfaces alternating with the stamens). Stigmas wet type, or dry type; papillate, or non-papillate; Group II type and Group IV type. Placentation ventral in the discrete ovaries. Ovules (1–)5–50 per locule (generally more or less numerous); pendulous; anatropous.
Fruit and seed features. Fruit non-fleshy; an aggregate (of two carpels), or not an aggregate (of one only, by abortion); dehiscent; comprising a pair of ‘follicles’, or commonly only one of the pair developing. Seeds endospermic. Endosperm oily. Seeds conspicuously hairy (with a terminal coma of long, silky hairs). Cotyledons 2. Embryo chlorophyllous (4/7); straight. Seedling. Germination phanerocotylar, or cryptocotylar.
Physiology, biochemistry. Aluminium accumulation not found. Photosynthetic pathway: C3 and CAM.
Geography, cytology, number of species. World distribution: widespread. 2000 species.
Economic uses, etc. Prized cultivated succulents or vines from Asclepias, Hoya, Araujia, Ceropegia, Stapelia, Caralluma, Decabelone etc.
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