Rubiaceae

Growth Form and Habit

Extremely diverse, including trees, shrubs, lianas (vines), perennial herbs, and annual herbs. Some species are epiphytic.

Leaves

Leaves are characteristically opposite or, less commonly, whorled. They are simple and usually have entire (smooth) margins. The defining vegetative feature is the presence of stipules, typically located on the stem between the petioles of the opposite leaves (interpetiolar stipules). These stipules vary greatly in form: they can be small and scale-like, large and leaf-like (making leaves appear whorled, as in Galium), fused into a sheath around the stem, or divided into bristles or glands.

Inflorescence

Inflorescence type is highly variable across the family. Commonly cymose (often complexly branched), but also includes panicles, dense heads (e.g., Cephalanthus, Morinda), fascicles (axillary clusters), or sometimes flowers are solitary.

Flowers

Flowers are usually bisexual (sometimes unisexual, plants then often dioecious), typically actinomorphic (radially symmetric), and most commonly 4- or 5-merous (parts in fours or fives, occasionally 3-10).

• Calyx: Composed of usually 4 or 5 sepals fused at the base (synsepalous) and situated on top of the ovary. Calyx lobes are often small or sometimes absent. In some genera (Mussaenda, Warszewiczia), one or more calyx lobes on some flowers may be greatly enlarged and brightly colored like petals (semaphylls).
• Corolla: Composed of usually 4 or 5 fused petals (sympetalous), forming a corolla tube of varying length and shape (funnelform, tubular, salverform, rotate, campanulate). Corolla lobes often valvate (meeting edge-to-edge), imbricate (overlapping), or convolute (twisted) in bud. Colors are diverse.
• Androecium: Stamens number as many as the corolla lobes (usually 4 or 5) and alternate with them. They are attached to the inside of the corolla tube (epipetalous). Anthers typically open via longitudinal slits.
• Gynoecium: Ovary is characteristically inferior (very rarely superior or half-inferior), composed of usually 2 fused carpels (syncarpous), forming typically 2 locules (sometimes 1 or more). Placentation is usually axile, with one to numerous ovules per locule. A single style, often slender, usually terminates in 2 stigma lobes or a capitate (head-like) stigma. Heterostyly (pin and thrum forms with reciprocal style and stamen lengths) is common in the family. A nectar disc is usually present on top of the ovary.

Fruits and Seeds

Fruit type is extremely variable and important for tribal classification within the family. Common types include:

• Capsule: Dry, dehiscent fruit (loculicidal or septicidal, e.g., Houstonia, Cinchona).
• Berry: Fleshy, indehiscent fruit with multiple seeds (e.g., Coffea, Gardenia, Psychotria).
• Drupe: Fleshy, indehiscent fruit with one or more hard stones (pyrenes), each containing a seed (e.g., some Psychotria).
• Schizocarp: Fruit that splits into single-seeded segments (mericarps) (e.g., Galium, Diodia).
• Less common types include nuts or samaras.

Seeds are variable, sometimes winged, with endosperm typically present.

Chemical Characteristics

The family is exceptionally rich in alkaloids, particularly indole alkaloids (e.g., quinine, cinchonine from Cinchona; emetine from Carapichea; mitragynine from Mitragyna) and purine alkaloids (caffeine from Coffea). Iridoid glycosides are also common. Anthraquinones are found in some groups (e.g., Rubia, source of red dyes). Saponins and tannins are also present. Unlike the related Apocynaceae, milky latex is generally absent.