Trees, shrubs, or subshrubs, usually synoecious, terrestrial, unarmed, occasionally clonal by root sprouts; young growth usually glandular, usually aromatic when crushed; trunk often with smooth or scaly bark; hairs unicellular, simple or dibrachiate. Leaves usually persistent, opposite, alternate, or whorled, sometimes decussate, simple; without true stipules; usually petiolate; blade leathery, papery, or submembranous, margins entire, sometimes somewhat sinuate. Inflorescences usually axillary, sometimes terminal or pseudoterminal, solitary flowers, dichasia, racemes, panicles, or spikes; bracts often present; bracteoles usually present. Flowers usually bisexual, rarely unisexual, (0–)4 or 5(–7)-merous, actinomorphic; usually epigynous, rarely semiepigynous; hypanthium obconic, cylindric, or cup-shaped, sometimes prolonged beyond summit of ovary; calyx lobes distinct and, usually, imbricate, or fused in calyptra that falls as a unit or tears open; petals distinct and imbricate, or fused with calyx, rarely coherent, usually equaling calyx lobes (when lobes distinct); nectary glands, when present, produced on disc surrounding style; stamens 10–720; anthers basifixed or dorsifixed, usually dehiscing by slits; pistil 1; ovary inferior (partially so in Melaleuca), 1–6[–18]-locular and carpellate; placentation axile, subapical, or basal; style 1; stigma 1; ovules 2–300(–500), usually biseriate or multiseriate, bitegmic. Fruits berries, capsules with apical dehiscence, or nutlike. Seeds 1–100+; seed coat membranous, ± leathery, or hard and bony; embryo starchy or oily; endosperm scant or absent.
sw, s, se United States, Mexico, West Indies, Central America, South America, s, se Asia, Africa, Pacific Islands (Hawaii, New Guinea, Philippines), Australia, nearly worldwide in tropical, subtropical, and Mediterranean regions.
Genera ca. 130, species ca. 6000 (13 genera, 38 species in the flora).
Myrtaceae are apparently of Gondwanan origin with centers of diversity in tropical America and Australasia and with fewer species in Africa and southern Asia. Syzygium aromaticum (Linnaeus) Merrill & L. M. Perry (clove) and Pimenta dioica (Linnaeus) Merrill (allspice) are economically important spices; Psidium guajava (guava) is a common tropical fruit; species of Eucalyptus are widely planted for fast growing timber and as ornamentals. Melaleuca (including Callistemon) and other genera are planted as ornamentals with M. quinquenervia having become an invasive pest in Florida.
Native and introduced genera of Myrtaceae in North America can be divided conveniently into two groups: those with dry fruit (Chamelaucium, Eucalyptus, Leptospermum, and Melaleuca) and those with fleshy fruit (Calyptranthes, Eugenia, Luma, Mosiera, Myrcianthes, Myrtus, Psidium, Rhodomyrtus, and Syzygium). This division based on fruit type has historically been the basis for recognizing subfamilies or tribes; molecular work shows that neither group is monophyletic. For the purposes of this treatment, the division based on fruit type will be retained, but without formal taxonomic standing. Among the fleshy-fruited genera, embryo structure has been taxonomically important. In the bony-seeded genera (Mosiera, Myrtus, Psidium, and Rhodomyrtus), the embryos are small and difficult to see. The C-shaped embryos in that group have small, leaflike or linear cotyledons equal to or shorter than the cylindrical hypocotyls. In other fleshy-fruited genera, it is usually possible to open the seed coat and see the embryo. In Eugenia, the embryo is mainly cotyledon tissue fused into a reniform to globose mass. In Myrcianthes and Syzygium, the cotyledons are similar to those of a bean and unfused and the hypocotyl is insignificant. In Calyptranthes, the cotyledons are leaflike and folded into a bundle and the equally long hypocotyl curls around the bundle. In Luma, the cotyledons are lenticular and pressed against each other and the hypocotyl about equals them in length.
Recently, Pimenta dioica has been shown to be naturalized near Miami, Florida. It is most similar to Syzygium cumini (both have berry fruits and many-flowered panicle inflorescences). The two species are compared under 2. S. cumini.
|1||Fruits capsules or nutlike; leaves on mature stems mostly alternate (opposite in Chamelaucium and Melaleuca linariifolia); seeds usually 1–3 mm.||> 2|
|2||Trees (sometimes large shrubs in E. conferruminata); perianth parts fused in calyptrae.||Eucalyptus|
|2||Shrubs or trees; perianth parts distinct.||> 3|
|3||Inflorescences dense cylindrical clusters of flowers surrounding stems apically or subapically, as a bottlebrush; stamens several times longer than perianth.||Melaleuca|
|3||Inflorescences clustered or flowers solitary, not forming bottlebrush; stamens about as long as perianth.||> 4|
|4||Leaves opposite (decussate); style with a ring of hairs just below stigma; ovaries 1-locular; fruits nutlike.||Chamelaucium|
|4||Leaves alternate; style without a ring of hairs just below stigma; ovaries 6–12-locular; fruits capsules.||Leptospermum|
|1||Fruits berries; leaves on mature stems mostly opposite, rarely subopposite or whorled; seeds often 3+ mm.||> 5|
|5||Calyces closed in bud or with porelike opening at apex (appearing closed in Syzygium cumini), persisting after anthesis as irregular parts or falling completely.||> 6|
|6||Inflorescences 1-flowered, or 3-flowered dichasia; seed coats bony.||Psidium (in part)|
|6||Inflorescences 3–100-flowered, mostly panicles; seed coats membranous or papery.||> 7|
|7||Petioles 2–8 mm; fruits spheroid to oblate (shorter than to as long as wide); calyptrae formed by connate calyx lobes; cotyledons thin, foliaceous (folded).||Calyptranthes|
|7||Petioles 10–20 mm; fruits ellipsoid or subglobose (longer than wide); calyptrae formed by coherent petals; cotyledons thick, plano-convex.||Syzygium (in part)|
|5||Calyces open in bud, lobes clearly distinguishable, persisting after anthesis as 4 or 5 distinct lobes.||> 8|
|8||Flowers 5-merous (sometimes 7-merous in Rhodomyrtus).||> 9|
|9||Petals pink or red; leaves with brochidodromous to acrodromous venation.||Rhodomyrtus|
|9||Petals whitish; leaves usually with brochidodromous venation, less often partially eucamptodromous.||> 10|
|10||Flower buds with calyx open before anthesis; calyx scarcely tearing, if at all, at anthesis; seed coats shiny, not notably dense, few cells thick, easily broken.||Myrtus|
|10||Flower buds with calyx closed completely or open only by apical pore before anthesis; calyx tearing irregularly at anthesis; seed coats dull, dense, many cells thick, broken only with difficulty.||Psidium (in part)|
|8||Flowers [3 or]4-merous (except sometimes 5-merous in Myrcianthes and Syzygium).||> 11|
|11||Inflorescences panicles.||Syzygium (in part)|
|11||Inflorescences solitary flowers, racemes, dichasia, fascicles, or umbel-like clusters.||> 12|
|12||Seeds 1–27, 3–6 mm; embryos.||> 13|
|13||Seed coats membranous; embryos lenticular; cotyledons about as long as hypocotyl; leaf blades apiculate to abruptly acuminate; California.||Luma|
|13||Seed coats bony; embryos Florida.||Mosiera|
|12||Seeds usually 1(–4), usually 10–15 mm; embryos each a solid, globose or reniform mass, or with 2 distinct, plano-convex cotyledons (beanlike).||> 14|
|14||Cotyledons connate, fused in a mass.||Eugenia|
|14||Cotyledons distinct, each thick, plano-convex, beanlike.||> 15|
|15||Inflorescences solitary flowers or dichasia; berries 6–15 mm; hypanthia not prolonged beyond summit of ovary, base not attenuate.||Myrcianthes|
|15||Inflorescences racemes; berries 14–40 mm; hypanthia prolonged beyond summit of ovary, base often attenuate.||Syzygium (in part)|