Rutaceae

Growth Form and Habit

Mostly trees and shrubs, typically evergreen or deciduous. Some are perennial herbs (Ruta, Boenninghausenia) or lianas. Many species are armed with thorns (modified stems) or prickles (epidermal outgrowths), especially Zanthoxylum and some Citrus.

Leaves

Leaves are usually alternate and spiral, but sometimes opposite. They can be simple or compound (pinnately, trifoliolate, or rarely palmately). A key diagnostic feature is the presence of scattered pellucid gland dots containing aromatic essential oils, visible as translucent spots when the leaf is held to the light. Leaves are typically aromatic when crushed. Stipules are usually absent (exstipulate).

Characteristic pellucid gland dots containing essential oils, visible in a Citrus leaf held to light.

Inflorescence

Inflorescence structure is variable, commonly cymose or flowers solitary. Racemes, panicles, or dense clusters (fascicles) also occur. Usually axillary or terminal.

Flowers

Flowers are usually bisexual (sometimes unisexual, plants then often dioecious), typically actinomorphic (radially symmetric), sometimes slightly zygomorphic. Flowers are usually 4- or 5-merous and hypogynous.

• Calyx: Typically 4 or 5 sepals, either distinct or fused at the base (connate).
• Corolla: Typically 4 or 5 distinct (free) petals (rarely fused or absent), often white, yellow, pink, or greenish, frequently somewhat fleshy or glandular.
• Androecium: Stamens variable in number, often twice as many as petals (e.g., 8 or 10) arranged in two whorls, sometimes numerous (15 to many, e.g., Citrus), or sometimes only as many as petals (e.g., 4 or 5). Filaments usually distinct, but sometimes fused basally or grouped into bundles (fascicled). Stamens typically arise from the base or outer edge of a prominent nectar disc. Anthers open via longitudinal slits.
• Gynoecium: Ovary is superior, composed of usually 4 or 5 (sometimes 1 to many) carpels. Carpels can be fused (syncarpous) usually forming an equal number of locules, OR sometimes distinct at the base but united by a common style or stigma (apocarpous to weakly syncarpous). Placentation is typically axile (sometimes parietal or basal), with usually 1 or 2 (sometimes numerous) ovules per locule. A prominent, annular or cushion-like nectar disc is usually present between the stamens and the ovary. Style(s) single and terminal, or sometimes separate styles; stigma(s) capitate or lobed.

Generalized Rutaceae flower showing 4-5 sepals/petals, often 8/10 or numerous stamens arising around a prominent nectar disc, and superior ovary.

Fruits and Seeds

Fruit type is exceptionally diverse across the family:

• Hesperidium: A specialized berry with a leathery, glandular rind (peel), partitioned fleshy interior filled with juice sacs (modified epidermal hairs). Unique to Citrus and close relatives.
• Drupe: Fleshy fruit with one or more hard stones (Amyris, Casimiroa, Zanthoxylum sometimes).
• Samara: Dry, winged fruit (Ptelea - wafer-like wing; Balfourodendron).
• Capsule: Dry, dehiscent fruit (Ruta).
• Follicle(s): Dry fruit splitting along one suture, often occurring in clusters derived from partially separate carpels (Zanthoxylum).
• Schizocarp: Fruit splitting into segments.
• Berry: Simple fleshy fruit (Murraya, Glycosmis).

Seeds sometimes winged or arillate. Endosperm present or absent; embryo often green.

Fruit diversity: Hesperidium (Citrus), Samara (Ptelea), Follicles (Zanthoxylum), Drupe (Amyris).

Chemical Characteristics

Defined by the presence of secretory cavities (gland dots) producing complex ethereal (essential) oils, rich in various terpenoids (especially bitter limonoids) and aromatic compounds. Also characteristic are coumarins (including photosensitizing furanocoumarins in some genera like Ruta) and various types of alkaloids (quinoline, acridone, imidazole). These compounds contribute to the family's aromas, flavors, medicinal properties, and toxicity.