Apocynaceae: The Dogbane Family of Flowering Plants
Apocynaceae is a large and diverse family of flowering plants that includes about 400 genera and 4,555 species of trees, shrubs, herbs, stem succulents, and vines. The family is distributed mainly in tropical and subtropical regions of the world, and many of its members are cultivated as ornamentals for their attractive flowers and foliage. The family name comes from the genus Apocynum, which means “dog-away” in Greek, because some species were used as dog poison in ancient times.
The family is characterized by having milky sap that is often poisonous, smooth-margined leaves that are usually opposite or whorled, and flowers that are typically bisexual, actinomorphic, and pentamerous. The flowers are often arranged in clusters called cymes or umbels. The corolla may have a tube or lobes, and sometimes an extra set of petal-like structures called a corona. The stamens are usually attached to the corolla tube and alternate with the lobes. The ovary is superior and consists of two carpels that are usually free from each other, except for being joined near their apex by the styles or stigmas. The fruit may be a berry, a capsule, or a follicle that splits open at maturity and releases many seeds that are often winged or tufted.
The family is divided into five subfamilies: Rauvolfioideae, Apocynoideae, Periplocoideae, Secamonoideae, and Asclepiadoideae. The latter subfamily was formerly treated as a separate family (Asclepiadaceae) and is distinguished by having pollen grains that are grouped into waxy masses called pollinia. The pollinia are attached to specialized structures called translators that help in the transfer of pollen from one flower to another by insects. The Asclepiadoideae also have a more complex gynoecium with a five-sided style head and a nectary disk at the base of the stamens.
Many members of the family have medicinal or economic importance. For example, Catharanthus roseus (Madagascar periwinkle) is a source of alkaloids that are used for treating leukemia and other cancers. Rauvolfia serpentina (Indian snakeroot) produces reserpine, which is used for lowering blood pressure and treating mental disorders. Apocynum cannabinum (Indian hemp) and A. androsaemifolium (spreading dogbane) have tough fibers that are used for making ropes and textiles. Nerium oleander (oleander) and Thevetia peruviana (yellow oleander) are widely grown as ornamental shrubs but are highly toxic if ingested.
Some examples of ornamental plants in the family are Vinca minor (periwinkle), Plumeria rubra (frangipani), Allamanda cathartica (golden trumpet), Mandevilla sanderi (Brazilian jasmine), Carissa macrocarpa (Natal plum), Hoya carnosa (wax plant), Asclepias tuberosa (butterfly weed), Stapelia gigantea (carrion flower), and Adenium obesum (desert rose).
The Apocynaceae family is also notable for its ecological interactions with animals, especially insects. Many species have evolved adaptations to attract, reward, or deceive pollinators. For example, some species have corollas that are shaped like tubes, funnels, or bells to accommodate the mouthparts of long-tongued insects such as bees, butterflies, or moths. Some species have corollas that are brightly colored or have contrasting patterns to guide the pollinators to the nectar or pollen. Some species have corollas that emit fragrant or foul odors to attract specific pollinators such as beetles or flies. Some species have corollas that mimic the appearance or scent of female insects to lure male insects for pseudocopulation.
Some species of Apocynaceae have developed mutualistic relationships with certain insects that feed on their sap or nectar and in return provide protection from herbivores or predators. For example, some species of Asclepias (milkweeds) are host plants for the larvae of monarch butterflies (Danaus plexippus), which sequester the toxic cardiac glycosides from the plant and become unpalatable to birds and other predators. Some species of Plumeria (frangipanis) are associated with ants that nest in their hollow stems and defend the plant from herbivorous insects. Some species of Rauvolfia (snakeroots) are visited by honeybees that collect their resin and use it to seal their hives and deter parasites.