Rosaceae

Growth Form and Habit

Highly diverse, including annual or perennial herbs (e.g., Potentilla, Fragaria), shrubs (Rosa, Rubus, Spiraea), and trees (Prunus, Malus, Pyrus, Crataegus). Some are climbers (Rosa) or scrambling (Rubus). Many woody species bear thorns (modified stems, Crataegus) or prickles (epidermal outgrowths, Rosa, Rubus).

Leaves

Leaves are typically arranged alternately and spirally (less commonly opposite). They can be simple (Prunus, Malus, Crataegus) or compound (pinnately or palmately, e.g., Rosa, Rubus, Potentilla, Sorbus). A key characteristic is the usual presence of paired stipules at the base of the petiole; these stipules vary from small and deciduous to large and persistent, sometimes fused to the petiole (Rosa). Leaf margins are very often serrated.

Inflorescence

Inflorescence type is variable; flowers may be solitary (Rosa in some cases) or borne in various clusters such as cymes, racemes, panicles, corymbs (often flat-topped clusters, e.g., Crataegus, Malus), or umbels.

Flowers

Flowers are usually bisexual, typically actinomorphic (radially symmetric), often showy, and characteristically possess a hypanthium (a floral cup or tube formed by the fused bases of sepals, petals, and stamens). Typically 5-merous perianth (sometimes 4-merous or numerous).

• Calyx: Typically composed of 5 distinct sepals attached to the rim of the hypanthium. An epicalyx (whorl of bractlets alternating with sepals) is present below the calyx in some herbaceous genera (e.g., Potentilla, Fragaria, Geum).
• Corolla: Typically composed of 5 distinct (free) petals, also attached to the hypanthium rim, often rounded or obovate, and frequently white, pink, or red (sometimes yellow). Petals sometimes absent or numerous (cultivated roses).
• Androecium: Stamens are usually numerous (often >15, sometimes fewer, e.g., 5 or 10), arranged in whorls, attached to the inner rim of the hypanthium. Filaments distinct; anthers open by longitudinal slits.
• Gynoecium: Highly variable, key to subfamily classification:
  • Ovary superior or inferior (depending on degree of hypanthium fusion).
  • Carpels 1 to many.
  • If 1 carpel (Prunus): superior ovary, simple pistil.
  • If multiple carpels: can be distinct (apocarpous, e.g., Rosa, Rubus, Spiraea) situated within or upon the hypanthium, OR fused (syncarpous, e.g., Malus, Pyrus) and often fused to the hypanthium (inferior ovary).
  • Ovules usually 1 or 2 per carpel (sometimes more). Styles distinct or fused.

Fruits and Seeds

Fruit diversity is immense and taxonomically important:

• Follicle(s): Dry fruit from one carpel splitting along one suture. Often occurs as an aggregate (Spiraea).
• Achene(s): Dry, indehiscent, single-seeded fruit. Can be numerous on a fleshy receptacle (Fragaria - strawberry, where fleshy part is receptacle) or enclosed within a fleshy hypanthium (Rosa - rose hip).
• Drupe: Fleshy fruit with a single hard stone (endocarp) enclosing the seed, derived from a single carpel (Prunus - cherry, plum, peach, almond).
• Aggregate of Drupelets: Numerous small drupes clustered together on a receptacle (Rubus - raspberry, blackberry).
• Pome: Fleshy accessory fruit developing from an inferior ovary where the fleshy tissue is primarily derived from the swollen hypanthium surrounding the core (Malus - apple, Pyrus - pear, Crataegus - hawthorn, Amelanchier - serviceberry).

Seeds usually lack endosperm; cotyledons often fleshy.

Chemical Characteristics

Often contain tannins and complex phenolic compounds. Cyanogenic glycosides (releasing cyanide upon damage) are common, particularly in seeds and leaves of Prunus species (e.g., amygdalin in almond/peach pits). Sorbitol (a sugar alcohol) is often present. Significant alkaloids are generally rare.