Apocynaceae
Dogbane Family / Milkweed Family
Apocynaceae is a large and diverse family of flowering plants, commonly known as the dogbane family. Following modern classifications (APG system), it also includes the plants formerly placed in the Asclepiadaceae family, now often referred to as the milkweed subfamily (Asclepiadoideae). This expansive family contains over 5,300 species in about 378 genera, exhibiting a wide range of habits including herbs, shrubs, trees, vines, and succulents. A hallmark feature of many members is the presence of milky latex.
Overview
The Apocynaceae family has a cosmopolitan distribution but is most diverse in tropical and subtropical regions. It is renowned for its chemical diversity, producing numerous potent compounds such as alkaloids and cardiac glycosides, making many species poisonous but also sources of important medicines (e.g., anti-cancer drugs from Catharanthus roseus). Many species are cultivated as ornamentals for their often showy and fragrant flowers (e.g., Plumeria, Nerium, Vinca).
Key characteristics often include simple, opposite or whorled leaves and the presence of milky sap (latex) when stems or leaves are broken. Flowers are typically 5-merous and radially symmetric, though often with complex structures. The inclusion of the former Asclepiadaceae introduced highly specialized floral features into the family concept, particularly the fusion of reproductive parts into a gynostegium and the aggregation of pollen into pollinia, often associated with intricate pollination mechanisms involving insects.
Ecologically, many Apocynaceae, especially in the Asclepiadoideae subfamily, play crucial roles as host plants for specific insects, most famously the monarch butterfly (Danaus plexippus) whose larvae feed exclusively on milkweeds (Asclepias species), sequestering cardiac glycosides for their own defense.
Quick Facts
- Scientific Name: Apocynaceae
- Common Name: Dogbane family, Milkweed family
- Number of Genera: Approximately 378
- Number of Species: Approximately 5,350
- Distribution: Cosmopolitan, most diverse in tropical and subtropical regions.
- Evolutionary Group: Eudicots - Asterids
- Order: Gentianales
Key Characteristics
Growth Form and Habit
Extraordinarily diverse, including annual and perennial herbs, subshrubs, shrubs, lianas (vines), small to large trees, and stem succulents (e.g., stapeliads). Most members contain milky or sometimes clear latex, which is often poisonous.
Leaves
Leaves are typically simple with entire margins. They are most commonly arranged oppositely or in whorls of 3 or more, but rarely alternate. Stipules are usually absent or reduced to small glands or fringes at the base of the petiole.
Inflorescence
Inflorescences are highly variable but commonly cymose (determinate), appearing terminal or axillary. They can be condensed into umbel-like structures (common in Asclepiadoideae) or form panicles, racemes, or solitary flowers.
Flowers
Flowers are typically bisexual, actinomorphic (radially symmetric), and 5-merous (parts in fives).
- Calyx: Five sepals, usually fused at the base, often with glands (colleters) inside at the base.
- Corolla: Five petals fused into a tube of varying length, with lobes that are often twisted in bud (convolute aestivation). The shape can be rotate, salverform (with a narrow tube and spreading limb, like Vinca), funnelform, campanulate, or urceolate.
- Corona: An appendage or series of appendages between the corolla and stamens or on the corolla itself is common, varying greatly in form. In Asclepiadoideae, the corona is often complex, typically consisting of hoods and horns.
- Androecium: Five stamens, alternate with the corolla lobes, inserted on the corolla tube. Filaments are usually short. Anthers are distinct or connivent (converging) around the style head, sometimes adhering to it.
- Gynoecium: Variable, typically composed of two carpels that may be distinct at the base (ovaries separate) but fused by the styles and/or stigmas, or fully fused into a single compound ovary. Ovary position is usually superior.
- Asclepiadoideae Specializations: In this subfamily, stamens and stigma are fused into a central structure called a gynostegium. Pollen grains are aggregated into waxy masses called pollinia (typically two per anther), connected by translator arms and a corpusculum (clip) to form a pollinarium, which is dispersed as a single unit by pollinators.
Fruits and Seeds
Fruit types are diverse, including follicles (often occurring in pairs, derived from the two separate carpels), capsules, berries, or drupes. Seeds are often flattened and frequently possess a coma (tuft) of long, silky hairs aiding in wind dispersal, especially common in species with follicular fruits (like Asclepiadoideae and Apocynoideae).
Chemical Characteristics
The family is rich in secondary metabolites. Milky latex is characteristic and often contains various alkaloids (e.g., indole alkaloids like vincristine, reserpine) and cardiac glycosides (cardenolides, like oleandrin). These compounds often make the plants toxic to herbivores and humans but are also sources of valuable pharmaceuticals.
Field Identification
Identifying Apocynaceae involves looking for a combination of features, especially latex, leaf arrangement, and flower/fruit characteristics.
Primary Identification Features
- Milky Latex: Break a leaf or stem; the presence of white (sometimes clear) sap is a strong indicator (though not exclusive to this family).
- Opposite or Whorled Leaves: Check the arrangement of leaves on the stem; most species have opposite or whorled leaves, usually simple and entire.
- 5-Parted Flowers: Look for flowers with 5 sepals, 5 fused petals (often twisted in bud), and 5 stamens.
- Follicle Fruits: Fruits are often pairs of slender pods (follicles) that split open along one side.
- Comose Seeds: Seeds, especially from follicles, frequently have a tuft of silky white hairs (coma) for wind dispersal (very common in milkweeds).
- Asclepiad Features: In the milkweed group, look for the unique gynostegium and complex corona structures in the flower's center. Pollinia might be visible if probed carefully.
Secondary Identification Features
- Corolla Shape: Note if the flower is pinwheel-like, funnel-shaped, bell-shaped, etc.
- Corona: Presence and shape of any extra structures (corona) within the flower.
- Habit: Is it a vine, erect herb, shrub, or tree?
- Glands at Leaf Base/Sepal Base: Small glands (colleters) are sometimes visible.
- Scent: Many species have fragrant flowers (e.g., Plumeria).
Seasonal Identification Tips
- Spring/Summer: Main flowering period for most species. Observe flower structure, latex, and leaf arrangement.
- Late Summer/Fall: Fruits develop and mature. Look for characteristic follicles splitting open to release comose seeds.
- Winter: Dried stalks with persistent open follicles might be visible in herbaceous species. Woody species show opposite/whorled branching. Latex is present year-round.
Common Confusion Points
- Euphorbiaceae (Spurge Family): Some spurges also have milky latex and sometimes opposite leaves. However, their flower structure is very different, often highly reduced and clustered into cyathia.
- Convolvulaceae (Morning Glory Family): Some vines have similarly shaped flowers (funnelform), but typically have alternate leaves and lack milky latex.
- Rubiaceae (Coffee/Bedstraw Family): Often have opposite or whorled leaves and stipules (interpetiolar stipules are key), but lack milky latex and have different flower/fruit details (usually inferior ovary).
- Asclepiadaceae: No longer a point of confusion, as this family is now included within Apocynaceae (as subfamily Asclepiadoideae).
Field Guide Quick Reference
Look For:
- Milky latex
- Opposite/whorled simple leaves
- 5-parted flowers
- Corolla lobes often twisted in bud
- Fruit often paired follicles
- Seeds often with silky coma
- (Asclepiads: Gynostegium, corona, pollinia)
Key Variations:
- Habit: Herb, shrub, tree, vine, succulent
- Flower shape highly variable
- Presence/absence/shape of corona
- Fruit type (follicle, capsule, berry, drupe)
- Seed dispersal mechanism
Notable Examples
Apocynaceae includes many well-known ornamentals, medicinal plants, and ecologically important species.
Asclepias syriaca
Common Milkweed
A robust perennial herb native to North America, famous as a primary host plant for monarch butterfly larvae. Features large, opposite leaves, copious milky latex, and distinctive globose umbels of pinkish-purple flowers with prominent hoods and horns (corona). Produces large, warty follicles filled with comose seeds.
Vinca minor
Lesser Periwinkle / Myrtle
A trailing evergreen subshrub or groundcover native to Europe, widely cultivated and naturalized. Has glossy dark green opposite leaves and characteristic solitary, violet-blue (sometimes white or purplish) salverform flowers with lobes appearing pinwheel-like (convolute in bud). Contains vinca alkaloids.
Nerium oleander
Oleander
An evergreen shrub or small tree native to the Mediterranean region, now widely grown in warm climates. Has leathery, lance-shaped leaves typically in whorls of three. Produces showy clusters of flowers (white, pink, red, yellow) often with a fringed corona. All parts are highly toxic due to cardiac glycosides like oleandrin.
Plumeria rubra
Frangipani
A deciduous shrub or small tree native to Central America and the Caribbean, famous for its intensely fragrant flowers used in leis. Has thick succulent stems with milky latex, large simple leaves clustered at stem tips, and terminal clusters of waxy, salverform flowers in various colors (white, yellow, pink, red). Leaves are alternate, an exception in the family.
Catharanthus roseus
Madagascar Periwinkle
An evergreen subshrub native to Madagascar, widely cultivated as an ornamental annual in cooler climates. Has opposite leaves and salverform flowers typically pink or white with a darker eye. It is the source of important anti-cancer alkaloids vincristine and vinblastine. Often confused with Vinca minor but is typically erect, not trailing.
Apocynum cannabinum
Indian Hemp / Dogbane
A perennial herb native to North America, representing the genus from which the family gets its name. It has opposite leaves, milky latex, and clusters of small, greenish-white, bell-shaped flowers. Produces slender paired follicles. Historically used by Native Americans for fiber (hence "Indian Hemp"). Contains cardiac glycosides.
Phylogeny and Classification
Apocynaceae is placed within the order Gentianales, part of the large asterid clade of eudicots. Its sister family within Gentianales is debated, but it shares the order with Gentianaceae, Rubiaceae, Loganiaceae, and Gelsemiaceae.
The modern concept of Apocynaceae is significantly broader than historical treatments, most notably incorporating the entire former family Asclepiadaceae as the subfamily Asclepiadoideae. This inclusion is strongly supported by molecular phylogenetic data. The family is typically divided into five subfamilies: Apocynoideae, Asclepiadoideae, Periplocoideae, Rauvolfioideae, and Secamonoideae. These subfamilies exhibit distinct combinations of floral structures, fruit types, and chemical profiles.
Position in Plant Phylogeny
- Kingdom: Plantae
- Clade: Angiosperms (Flowering plants)
- Clade: Eudicots
- Clade: Asterids
- Order: Gentianales
- Family: Apocynaceae
Evolutionary Significance
Apocynaceae showcases remarkable evolutionary diversification in growth form, chemical defense, and reproductive strategies. The evolution of milky latex containing potent secondary metabolites represents a key defense against herbivory. The family displays a range of pollination mechanisms, culminating in the highly specialized systems within Asclepiadoideae involving pollinia and insect-specific trapping mechanisms, representing a pinnacle of co-evolution between plants and pollinators. The frequent development of comose seeds facilitated long-distance wind dispersal, contributing to the family's wide distribution.
