Malvaceae

Growth Form and Habit

Extremely diverse: annual or perennial herbs, subshrubs, shrubs, lianas, and small to very large trees (e.g., Tilia, Ceiba, Adansonia - Baobabs).

Hairs and Sap

Plants are frequently covered in stellate (star-shaped) hairs, sometimes mixed with simple or glandular hairs, or scales. Tissues commonly contain mucilage canals or cells, resulting in slimy sap or texture when crushed (especially noticeable in mallows, okra, linden).

Leaves

Leaves are usually alternate and spiral, typically simple, but often palmately lobed or veined (sometimes digitately compound, e.g., Ceiba). Margins are commonly entire or toothed. Stipules are usually present but often fall off early (deciduous).

Often alternate, palmately veined/lobed leaves (left) and characteristic stellate hairs (right, magnified).

Inflorescence

Inflorescences are highly variable, often complex cymose structures borne in leaf axils or terminally. Can also be racemes, panicles, fascicles, or flowers solitary. An epicalyx (a whorl of bracts immediately below the true calyx) is present in many genera of subfamily Malvoideae (e.g., Hibiscus, Malva, Gossypium) but absent in others (e.g., Tilia, Theobroma, Abutilon, Sida).

Flowers

Flowers are usually bisexual, typically actinomorphic (radially symmetric), sometimes slightly zygomorphic. Usually 5-merous perianth (sometimes 4-merous or absent).

• Calyx: Typically 5 sepals, distinct or fused at the base (synsepalous). Often valvate (meeting edge-to-edge) in bud and persistent. May be subtended by an epicalyx (Malvoideae).
• Corolla: Typically 5 distinct (free) petals (sometimes absent), often large and showy. Petals frequently convolute (twisted or overlapping like a rolled-up flag) in bud and attached to the base of the staminal column (if present).
• Androecium: Stamens are usually numerous (ancestrally), but sometimes reduced (e.g., 5). A defining feature of subfamilies Malvoideae and Bombacoideae is the fusion of stamen filaments into a staminal tube or column (monadelphous) surrounding the style. The anthers are often borne along the upper part or apex of this tube and are typically kidney-shaped with only one locule (pollen sac) due to developmental division. In other subfamilies (representing former Tiliaceae, Sterculiaceae), stamens may be numerous and free, or grouped into bundles, sometimes alternating with staminodes.
• Gynoecium: Ovary is superior, composed of 2 to many (often 5 or 10) fused carpels (syncarpous). The number of locules usually equals the number of carpels. Placentation is typically axile, with 1 to numerous ovules per locule. A single style usually passes through the staminal tube (if present) and branches above, typically with as many stigma lobes or branches as carpels. Stigmas variable (capitate, linear, discoid).

Characteristic flower (Malvoideae type): 5 distinct petals, monadelphous staminal column surrounding style, superior ovary, often epicalyx below calyx.

Fruits and Seeds

Fruit type is highly diverse across the expanded family:

• Capsule: Dry, dehiscent fruit (loculicidal or septicidal). Common in Hibiscus, Gossypium (cotton boll), Abelmoschus (okra), Theobroma (cacao pod - technically a drupe-like capsule).
• Schizocarp: Fruit splitting into single-seeded segments (mericarps), often arranged in a ring or disc (Malva, Abutilon, Sida).
• Nut or Samara: Dry, indehiscent fruit. Small nutlets attached to a wing-like bract in Tilia.
• Berry or Drupe: Fleshy fruits occur less commonly.
• Large woody capsules also occur (Ceiba, Ochroma).

Seeds are sometimes covered in hairs (e.g., cotton fibers on Gossypium seeds) or winged. Endosperm present or absent, often oily.

Chemical Characteristics

Mucilage canals and cells are widespread and characteristic, leading to slimy sap. Stellate hairs are common. Cyclopropenoid fatty acids are found in seed oils of some groups (can be toxic). Flavonoids and terpenoids are common. Purine alkaloids like caffeine and theobromine are notable in Theobroma (cacao) and Cola (kola nut).