Lythraceae

J. Saint-Hilaire
Common names: Loosestrife Family
Treatment appears in FNA Volume 10.

Herbs, annual or perennial, shrubs, subshrubs, or trees, terrestrial, amphibious, or aquatic, usually unarmed (armed in Punica), rarely glaucous, clonal or not. Stems erect, decumbent, lax, spreading, creeping, trailing, floating, or submerged, young ones often 4-angled. Leaves deciduous, usually opposite, sometimes alternate, subalternate, or whorled, simple, rarely dimorphic, emergent and submerged leaves dissimilar; stipules usually absent; sessile, subsessile, or petiolate; blade membranous [leathery], venation brochidodromous, each marginal vein forming a series of loops, margins entire (except coarsely toothed in Trapa), trichomelike glands present in axil at petiole base. Inflorescences indeterminate or determinate, terminal or axillary, racemes, spikes, cymes, panicles, [thyrses], or flowers solitary; bracts absent (except present in Cuphea aspera); bracteoles present, paired on pedicels. Flowers bisexual [unisexual], actinomorphic or zygomorphic; perianth and androecium usually perigynous (semi-epigynous to epigynous in Punica, perigynous to semi-epigynous in Trapa), mono-, di-, or tristylous; floral tube or cup persistent, campanulate, turbinate, urceolate, cylindrical, or obconic, green (except red or yellow in some Cuphea and Punica), sometimes conspicuously ribbed; sepals persistent, 4–8, valvate, usually deltate, sometimes subulate in Lythrum, alternating with segments of epicalyx or epicalyx absent; petals usually caducous or deciduous, rarely persistent, 0–8, distinct, on interior margin of floral tube between sepals, crinkled, pinnately-veined; nectary present as distinct organ, as nectariferous tissue in wall of ovary or floral tube, or absent; stamens (1–)4–42[+], usually equal to or 2 times number of petals, usually in 2 whorls of unequal length, sometimes 1 whorl; anthers versatile [or basifixed], introrse, 2-locular, longitudinally dehiscent; pistil 1; ovary usually superior (semi-inferior to inferior in Punica and Trapa), 2–6-locular (to 9 twisted locules in Punica); placentation axile, placenta elongate or globose; septa incomplete near ovary apex (reduced to thin threads in Cuphea); style 1; stigma 1, capitate to punctiform, dry (wet in Heimia, Lagerstroemia, and Punica); ovules 1–1400, anatropous, bitegmic. Fruits capsules, walls usually thin and dry (thick in Decodon, woody in Lagerstroemia), or leathery berries (Punica), or indurated, 2–4-horned drupes (Trapa), usually smooth (striate in Rotala), dehiscence loculicidal, septicidal, or septifragal, or indehiscent and splitting irregularly. Seeds 3–250(–1400), usually brown (wine in Punica) [black, red], narrowly obovoid to fusiform, oblong to pyramidal, obovoid-semiovoid, semiorbicular, suborbicular, or elliptic (in outline), usually dry, fleshy in Punica, winged or not; endosperm not developed; embryo straight, oily, cotyledons usually ± complanate, rarely rolled, often auriculate or cordate.

Distribution

North America, nearly worldwide, mostly in tropical and subtropical areas.

Discussion

Genera 28, species ca. 600 (10 genera, 32 species in the flora).

Lythraceae are widely distributed in both hemispheres, generally in wet habitats, with limited temperate representation.

Taxa of Lythraceae in the flora area range in elevation from coastal plain to mid montane. Elevational ranges of the species are approximations based on regional topography taken from collection data where possible.

Lythraceae (order Myrtales) have recently been expanded to include the previously recognized families Duabangaceae, Punicaceae, Sonneratiaceae, and Trapaceae. Together they form a monophyletic family sister to Onagraceae (S. A. Graham et al. 2005). The traditional, more narrowly defined family (E. Koehne 1903) was divided into two tribes and four subtribes, a classification not supported by morphological or molecular phylogenetic evidence (Graham et al. 2011). Lythraceae are notably among only 28 angiosperm families that have evolved a heterostylous breeding system and one of three families in which the tristylous condition has evolved (S. C. H. Barrett et al. 2000). Six genera (Adenaria, Ammannia, Decodon, Lythrum, Pemphis, and Rotala) have distylous and/or tristylous species. Seed coats of many of the genera possess mucilaginous trichomes in the outermost cell layer that evaginate upon wetting, coming to resemble fungal growth. In Cuphea and in some genera outside the flora area, the trichomes reach half the length of the seeds and are spirally twisted. Floral merosity is highly variable in the family, and it is common to find a range of merosity in flowers of a single plant or within a population. The most frequent states are 4- and 6-merous. Only Decodon is commonly 5-merous. Submerged portions of the stems of the marsh-inhabiting genera Ammannia, Decodon, Lythrum, and Trapa can develop external spongy, aerenchymatous tissue that enhances oxygen exchange to the interior.

Some genera are widely cultivated: Cuphea and Lagerstroemia as landscape and garden plants, Punica for the pomegranate fruit, and Lawsonia for the leaves from which henna dye is obtained. Trapa and some species of Lythrum and Rotala can be invasive and pose significant problems for wetlands and river systems in North America.

Selected References

None.

Key

1 Leaf blade margins coarsely toothed in distal 1/2; fruits drupes, with 2–4 hardened, spiny horns; rosette-forming, rooted or floating aquatic herbs. Trapa
1 Leaf blade margins entire; fruits capsules or berries, without spines or horns; aquatic, amphibious, or terrestrial herbs, shrubs, subshrubs, or trees. > 2
2 Branches often terminating as indurate thorns; floral tubes semi-epigynous to epigynous; fruits berries, leathery, crowned by persistent sepals and stamens; seeds fleshy. Punica
2 Branches without thorns; floral tubes perigynous; fruits capsules, enclosed by persistent floral tube; seeds dry. > 3
3 Stamens 36–42, 3+ times number of sepals; seeds unilaterally winged; shrubs or small trees. Lagerstroemia
3 Stamens (1 or)2–12(–27) usually equal to or 2 times number of sepals; seeds not winged; herbs, shrubs, or subshrubs. > 4
4 Floral tubes cylindrical or obconic; flowers actinomorphic or zygomorphic. > 5
5 Flowers zygomorphic; capsule dehiscence by longitudinal complementary slits in wall and floral tube, placenta and seeds ultimately exserted. Cuphea
5 Flowers actinomorphic; capsule dehiscence septicidal or septifragal, placenta and seeds remaining within capsules. Lythrum (in part)
4 Floral tubes campanulate, urceolate, or turbinate; flowers actinomorphic. > 6
6 Shrubs or subshrubs, (5–)10–30 dm; petals rose purple or bright yellow, 5–15 mm. > 7
7 Inflorescences simple or compound cymes, 3+-flowered; flowers pedicellate, petals rose purple, 8–15 mm. Decodon
7 Inflorescences leafy racemes; flowers sessile or subsessile, petals bright yellow, 5–10(–14) mm. Heimia
6 Herbs, 0.4–10 dm; petals deep to pale purple, rose purple, rose, pink, lavender, or white, 1–4(–7) mm, or petals absent. > 8
8 Capsules septicidally dehiscent, walls finely, transversely striate (10×); inflorescences terminal or axillary racemes, axillary spikes, or solitary flowers. Rotala
8 Capsules indehiscent, splitting irregularly or circumscissile then splitting irregularly, walls smooth; inflorescences axillary cymes, axillary or terminal racemes (sometimes spikelike in Lythrum), or solitary flowers. > 9
9 Leaf blade bases cordate or auriculate; floral tubes campanulate or urceolate, semiglobose to globose in fruit; sepals to 1/4 floral tube length. Ammannia
9 Leaf blade bases rounded, cordate, tapered, attenuate, or truncate; floral tubes broadly campanulate; sepals 1/3–1/2 floral tube length. > 10
10 Sepals 4; petals 0; epicalyx segments absent. Didiplis
10 Sepals 6; petals (0 or)6; epicalyx segments present, equal to 2 times longer than sepals. Lythrum (in part)