LEMNACEAE (Duckweed Family)
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.