Erect, creeping, scrambling or twining herbs and shrubs, sometimes succulent, usually with milky latex. Leaves opposite, simple, sometimes reduced or absent, exstipulate, entire. Flowers actinomorphic, bisexual, 5-meous, in terminal or lateral umbelloid, corymbose or paniculate cymes. Sepals united, shortly connate or almost free, imbricate. Corolla gamopetalous, rotate, campanulate, tubular, lobes valvate or twisted in bud, rarely imbricate. Stamens 5, epipetalous, alternating with corolla lobes, either connivent by the anthers (Periplocoideae) or completely connate and adnate to the style apex forming a gynostegium (Asclepiadoideae). Corona of one or two series, attached to the corolla or to the staminal column or the outer series to the corolla and the inner to the staminal column. Anthers longitudinally dehiscent or opening by terminal slits. In Periplocoideae the pollen grains are in tetrads and fall into spatulate pollen carriers with adhesive disc formed on the surface of the style apex between the anthers. In Asclepiadoideae 1 or 2 pollinia (waxy masses) per anther locule are present; the pollinia of adjacent locules of adjacent anthers are connected by the translator apparatus, which consists of a corpusculum and two arms or retinaculae, each of which bears a pollinium. Carpels 2, superior, united only by the enlarged style apex, multiovulate, placentation parietal. Fruit of 2 distinct or single (by abortion) follicles. Seeds flattened, without an aril, comose.
A family with 175-180 genera and 2200 species (Willis, Dict. Fl. Pl. Ferns, 98.874. 1973) distributed mainly in the tropical and subtropical regions of the world; represented in Pakistan by 23 genera and 41 species. Opinion seems to be divided about segregating Periplocaceae from Asclepiadaceae. For instance Bullock (in Hutchinson & Dalziell, Fl. Trop. Afr. ed. 2. 2:81. 1963) and Huber (in Abeywickrama, Fl. Ceylon 1:28. 1973) have treated the two as independent families, whereas, more recently, Markgraf (in Tutin et al., Fl. Europ. 3:70. 1972) Rechinger f. (Fl. Iran. 73: 1. 1973), Spellman (in Ann. Miss. Bot. Gard. 62:103, 108. 1975) and Stevens (in Saldanha & Nicolson, Fl. Hassan Distr. 437. 1976) have taken a broader view and have accepted the family Asclepiadaceae including the subfamily Periplocoideae. We have also accepted the family in a broader sense.
i) Subfamily Asclepiadoideae
a. Tribe Asclepiadeae : Asclepias, Calotropis, Cynanchum, Glossonema, Holostemma, Oxystelma, Pentatropis, Pergularia, Sarcostemma, Vincetoxicum.
b. Tribe Marsdenieae : Hoya, Marsdenia, Telosma, Tylophora, Wattakaka.
c. Tribe Ceropegieae : Caralluma, Ceropegia, Leptadenia, Orthanthera.
ii. Subfamily Periplocoideae
Tribe Periploceae : Cryptolepis, Cryptostegia, Hemidesmus, Periploca.
Acknowledgements: I am grateful to the authorities of the following herbaria for herbarium and library facilities: Royal Botanic Gardens, Kew; British Museum (Natural History), London and National Herbarium, Islamabad. I am indebted to Mr. B.L. Burtt, Mr. I.C. Hedge (Edinburgh) and Mr. David Field (Kew) for going through the manuscript and offering suggestions for its improvement. I am thankful to Miss Surayya Khatoon who has kindly executed some of the illustrations. The financial assistance received from the United States Department of Agriculture under P.L. 480 with the coordination of the Pakistan Agricultural Research Council, Islamabad, is thankfully acknowledged.