LYTHRACEAE (toothcup family)

Plants annual or perennial herbs, less commonly shrubs (small trees elsewhere). Stems usually branched, sometimes 4-angled. Leaves opposite or less commonly alternate or whorled, variously simple, sessile or less commonly short-petiolate, the leaf blade unlobed and with entire margins. Stipules absent. Inflorescences of solitary or clustered axillary flowers, sometimes appearing as elongate terminal spikes or racemes with leaflike bracts. Flowers perfect, strongly perigynous (the hypanthium appearing as a calyx tube), actinomorphic or less commonly zygomorphic, subtended by a pair of minute bractlets. Calyces of 4–7 small, ascending, triangular, toothlike sepals at the tip of the calyxlike hypanthium, these sometimes alternating with small appendages, the hypanthium and calyx usually persistent at fruiting. Corollas of 4–7 petals (absent in Didiplis), these sometimes appearing wrinkled in bud, attached on the inner surface of the hypanthium near its tip, alternating with the sepals, usually not persistent at fruiting. Stamens 4–14, as many as or twice as many as the sepals, the filaments sometimes in 2 series or of 2 or 3 different lengths in different flowers, attached to the inner surface of the hypanthium toward its base, the anthers small, attached near the midpoint, yellow or less commonly brown or purple. Pistil 1 per flower, composed of 2–4(–6) fused carpels, the superior ovary sometimes with a swollen nectar disc (this sometimes incomplete or absent) at the base, the style 1, slender, ranging from very short to relatively long and exserted, persistent at fruiting, sometimes of 2 or 3 different lengths in different flowers, the stigma usually capitate. Ovules several to numerous. Fruits capsules, dehiscing irregularly or longitudinally between the locules (indehiscent in Didiplis). Seeds 3 to numerous, small, sometimes winged. About 30 genera, about 600 species, nearly worldwide, most diverse in tropical and warm-temperate regions.

Some members of the Lythraceae (in Missouri principally the genera Decodon and Lythrum) have a condition known as tristyly, in which the relative staminal filament and style lengths differ in different flowers on the same plant. This phenomenon was first studied in Lythrum salicaria (and other unrelated species) by Charles Darwin (1877). Some flowers (so-called pin flowers) have a relatively long style and short filaments, such that the anthers are positioned well below the stigma. Other flowers (so-called thrum flowers) have the reverse situation, with a relatively short style and long filaments, such that the anthers are positioned well above the stigma. This mechanism promotes cross-pollination between flowers. A third flower type, intermediate between the other two with the stamens positioned near the stigma, is produced less frequently and is more often self-pollinated. The three flower types also differ in relative sizes of pollen grains, with those of thrum flowers the largest (Mulcahy and Caporello, 1970). In some members of the Lythraceae, the situation is complicated by the fact that individual flowers have stamens of two of the three possible lengths.

Some species of Cuphea, Lagerstroemia L. (crepe myrtle), and Lythrum are commonly cultivated as ornamentals. Paradoxically, the exotic Lythrum salicaria also becomes naturalized and invasive in North American wetlands (for further discussion, see the treatment of that species). Similarly, the Eurasian Trapa natans L. (water chestnut), a mostly submerged aquatic that is sometimes cultivated in ponds, has invaded North American ponds, lakes, streams, and rivers. The genus Punica L. (pomegranate) is often included in the family and is cultivated for its fruits that are popular fresh (the red pulp is mainly from the fleshy seed coats) or as juice. The juice sometimes also is used as a colorant in other beverages and foods. Some species of Cuphea are being investigated as an oilseed crop and for potential medical uses in weight reduction and control of cholesterol.