Myrtaceae

Growth Form and Habit

Mostly evergreen trees and shrubs, ranging from small shrubs to some of the tallest flowering plants (Eucalyptus regnans). Rarely lianas or deciduous. Bark often distinctive, being smooth and peeling in strips or flakes (Eucalyptus, Psidium), fibrous (Melaleuca), or persistent and rough.

Leaves

Leaves are usually simple and typically have entire (smooth) margins. Arrangement is commonly opposite, but frequently alternate or whorled in some major groups (e.g., many adult Eucalyptus). Leaves are often leathery or hard (sclerophyllous). A key diagnostic feature is the presence of scattered pellucid gland dots containing aromatic essential oils; these appear as tiny translucent spots when the leaf is held up to the light. Stipules are usually absent or very small and quickly deciduous. Heterophylly (distinct juvenile vs. adult leaf forms) is common, especially in Eucalyptus. Venation is pinnate, and often a distinct intramarginal vein runs parallel to and just inside the leaf edge.

Characteristic Myrtaceae leaf features: Pellucid gland dots (visible against light) and often an intramarginal vein.

Inflorescence

Inflorescence structure is highly variable. Flowers may be solitary or arranged in axillary or terminal cymes, racemes, panicles, dense heads, or characteristic spike-like "bottlebrushes" where flowers with prominent stamens are arranged around the stem (Callistemon / Melaleuca).

'Bottlebrush' inflorescence typical of Callistemon/Melaleuca, with showy stamens.

Flowers

Flowers are usually bisexual and typically actinomorphic (radially symmetric). Perianth parts (sepals and petals) are usually 4- or 5-merous, but stamens are almost always numerous. A hypanthium (floral tube) is usually present, fused to the ovary.

• Calyx: Usually 4 or 5 sepals, distinct or fused basally, sometimes highly reduced. Attached to the rim of the hypanthium above the inferior ovary. In many Eucalyptus and related genera, the sepals (and petals) are fused together to form a cap called an operculum (or calyptra) that covers the bud and detaches as the flower opens.
• Corolla: Usually 4 or 5 distinct (free) petals, attached to the hypanthium rim. Petals are often small, rounded, inconspicuous compared to the stamens, and may fall early. Sometimes petals are absent or fused into the operculum (Eucalyptus).
• Androecium: Stamens are characteristically numerous (usually >10, often hundreds), arranged in multiple whorls, and attached to the inner rim of the hypanthium. They are typically the most conspicuous part of the flower, with long, often brightly colored filaments (white, yellow, pink, red, purple). Filaments can be distinct or fused basally into bundles or a short tube. Anthers are usually small, versatile or basifixed, opening via longitudinal slits or sometimes apical pores.
• Gynoecium: Ovary is typically inferior (sometimes half-inferior or rarely superior), composed of usually 2-5 (sometimes more) fused carpels, forming as many locules. Placentation is usually axile (sometimes parietal), with typically numerous (sometimes few or one) ovules per locule. A single, usually elongated style arises from a depression on top of the ovary, topped by a small capitate or slightly lobed stigma. A nectar disc is often present on top of the ovary or lining the hypanthium.

Generalized Myrtaceae flower showing small petals (or operculum), numerous conspicuous stamens, and inferior ovary.

Fruits and Seeds

Fruit type is variable and divides the family into major groups:

• Capsule: Dry, dehiscent fruit, often woody, typically opening by valves or pores at the top (loculicidal). Characteristic of the fleshy-fruited tribes derived from them include Myrteae (Myrtus, Psidium, Eugenia).
• Drupe: Fleshy fruit with one or few hard stones containing seeds. Less common, found in some groups.
• Rarely a nut.

Seeds are usually numerous in capsules, fewer or one in berries/drupes. Variable in shape, sometimes winged. Endosperm is scant or absent.

Chemical Characteristics

The family is defined by the abundance of ethereal (essential) oils, primarily complex terpenoids (like cineole/eucalyptol, pinene, limonene, myrcene) and phenylpropanoids (like eugenol), stored within internal glandular cavities (visible as pellucid dots in leaves). These oils contribute to characteristic aromas and have ecological roles (defense, allelopathy, attracting pollinators) and economic uses. Tannins and various phenolic compounds are also common. Plants often accumulate aluminum. Lack iridoids, glucosinolates, and typically lack significant alkaloids.