ASCLEPIADACEAE (Milkweed Family)

Plants perennial herbs (woody elsewhere), lacking tendrils, usually with white latex, the sap thus appearing milky (except in Cynanchum and Asclepias tuberosa). Stems sometimes twining. Leaves opposite or less commonly alternate or whorled, simple. Stipules absent or minute, linear, and shed during leaf development. Leaf blades variously shaped, the margins entire or somewhat undulate, sometimes curled under. Inflorescences umbels, these solitary or in clusters of 2–4, axillary and/or terminal. Flowers perfect, hypogynous, actinomorphic, without subtending bracts. Calyces 5-lobed nearly to the base, often with minute glandular or scalelike projections near the tip of the tube between the lobes, often persistent at fruiting, the lobes spreading or reflexed. Corollas usually deeply 5-lobed, spreading, reflexed, or less commonly erect, spirally twisted in bud. Stamens 5, the filaments fused to the corolla tube and united into a columnar sheath around the carpels, the anthers fused to and forming a headlike structure with the stigmatic complex, with a corona of 5 variously shaped petaloid outgrowths covering the anther sacs (and often also the rest of the stamens and stigmas). Staminodes absent. Pollen grains of each of the 2 anther locules fused into a saclike mass (pollinium), the rightmost pollinium of each anther united with the lefthand pollinium of the adjacent anther via a short connecting arm (translator). Pistils 2 per flower, each of 1 carpel, these free at the ovary and style, but the stigmas (and anthers) fused into a relatively massive, 5-lobed or angled, headlike structure. Each ovary more or less superior, with 1 locule, the placentation parietal. Ovules numerous. Fruits follicles, these sometimes paired, variously shaped. Seeds variously shaped, usually flattened and winged, usually with a tuft of long, silky hairs at the tip. Fifty to 250 genera, 2,000–3,000 species, worldwide.

The flowers of the Asclepiadaceae are complex and have developed specialized structures to promote outcrossing by various insect pollinators. A specialized terminology has developed to account for these unusual floral structures. The complex of carpels, stamens, and coronas is collectively termed the gynostegium (Pl. 220 c). The pollinia from adjacent anthers are connected by an elaborate, acellular, wishbone-shaped complex consisting of 2 threadlike arms, called translators, and a central, longitudinally grooved, disklike structure called the corpusculum. The complex of 2 pollinia along with the translator apparatus is technically known as a pollinarium (Pl. 220 d), but in practice many botanists continue to refer merely to pollinia (pollinium, singular) in discussing the pollen transfer of asclepiad flowers. The corona may develop from the base of the staminal tube or from the tips of the anthers.

The space between adjacent corona segments, as well as that between the small, outwardly pointed, winglike flaps of adjacent anthers, forms a slot that guides the legs (and sometimes other body parts) of pollinators to become entangled in the translator complex of the pollinarium (with its pair of pollinia). Pollinators often struggle to remove their trapped legs from the flowers. Occasionally, various Asclepiadaceae are encountered with dead insects trapped in the flowers, having been unable to extract body parts from the slots, and moths have even been noted dangling by their proboscises. To effect pollination, the hapless pollinator that wrestles free of a flower must then visit a second flower and have its leg guided into a slot on the next gynostegium, where, if the pollinarium has previously been removed, a sticky stigmatic region in the chamber below the slot traps pollen from the pollinia, leading to pollination. With such a specialized pollination mechanism, it is easy to understand why most asclepiads produce few fruits relative to the large number of flowers.

The families Asclepiadaceae and Apocynaceae are treated here in the traditional sense, as they have been in most floristic works (Steyermark, 1963; Hartman, 1986a,; Hartman 1986b; Gleason and Cronquist, 1991). However, botanists have long accepted a close relationship between these groups. In recent decades, a number of authors (summarized in Rosatti, 1989; Judd et al., 1994, 2002) have argued that the unique floral characters of the Asclepiadaceae represent a syndrome of specializations within the Apocynaceae and, as such, the Asclepiadaceae should more accurately be classified as a subfamily or tribe of the latter family. Rosatti (1989) noted, however, that even those botanists who combine the two families continue to recognize the milkweeds as distinct at some level and that, especially in temperate floras, there is utility in continuing to recognize two separate families.

The generic classification adopted here follows that of Woodson (1941), who accepted relatively few, broadly circumscribed genera of North American Asclepiadaceae. Although American botanists during the last few decades have, with minor quibbles, almost universally embraced Woodson’s concepts, those in other parts of the world have tended to treat the family as it occurs in various regions as consisting of many more, much more narrowly circumscribed genera. The ultimate disposition of most of these segregates awaits future comprehensive investigations involving studies of these complexes on a worldwide basis.