LEMNACEAE (Duckweed Family)

(Landolt, 1986)

Small, floating or submerged aquatics, sometimes on wet soil. Roots present and unbranched or absent. Stems and leaves reduced to a flat or globose structure known as a thallus or frond (the latter term is used in the present treatment). Fronds with 0–16 veins, usually with 1 to several raised points on upper surface (papules), reproducing asexually by formation of plantlets from vegetative buds located in 1 or 2 pouches along the margin or upper surface. Pouches also containing the flowers. Inflorescences reduced, consisting of 1 or 2 staminate and 1 pistillate flowers, these sometimes with a tiny membranous sheath at base. Flowers rarely produced in nature, minute, unisexual. Calyces and corollas absent. Staminate flowers with 1 stamen. Pistillate flowers with 1 carpel consisting of a 1-locular ovary with 1–8 ovules, 1 short style, and small stigma. Fruits 1(–5)-seeded utricles. Seeds usually ribbed. Four genera, 34 species, worldwide.

The Lemnaceae provide considerable challenges for species determination. Flowers and fruits contain reliable characters for separating the various species but are rarely observed. Species generally tend to flower during late spring and summer, but flowering is so infrequent that months of bloom are not indicated for each species below. Plants should be keyed while fresh for observations of frond shape and papules. However, strong backlighting is necessary to observe the number and position of the veins. Placing a frond in red stain on a microscope slide helps to provide contrast for viewing veins. In spite of a detailed, recent monograph (Landolt, 1986) there is still controversy about generic and specific delimitations in the family. Some authors also interpret the flowers as bisexual structures, each one equivalent to an inflorescence as discussed above.

Species of Lemnaceae are important foods for wildlife, particularly waterfowl. Most Lemnaceae overwinter underwater or on the water surface as small “resting fronds” that are similar to those produced during the growing season, but a few species produce specialized, globose, resting structures, turions, at the end of the growing season, which overwinter underwater before resprouting the next spring.