keys to the vascular plants of east texas - Botanical Research ...

718 LEMNACEAE
Lemnaceae are tiny and extremely reduced morphologically, which made it difficult in the
past to determine the phylogenetic relationships of the family. Kvacek (1995) suggested that the
fossil genus Limnobiophyllum is a fossil link between the Araceae and the Lemnaceae, and
Stockey et al. (1997), using a cladistic approach and material of Limnobiophyllum, concluded
that Pistia (a free-floating aquatic member of the Araceae) plus Limnobiophyllum and
Lemnaceae form a monophyletic group. Indeed, this linkage of Lemnaceae to Araceae goes
back over 175 years (Hooker and Brown in Smith 1824; see discussion in Les et al. 2002). In addition,
molecular studies have linked Lemna with Pistia (Araceae) (Duvall et al. 1993b) or more
recently to Araceae subfamily Aroideae (French et al. 1995). In fact, the Lemnaceae is considered
by many authorities to have evolved from within the Araceae by extreme reduction, and it
has been suggested that the Lemnaceae be reduced to a subgroup within a more inclusive
Araceae (Mayo et al. 1995, 1997, 1998; Stockey et al. 1997; APG II 2003). From a cladistic standpoint,
the Araceae (without Lemnaceae) are thus paraphyletic and inappropriate for formal
recognition. However, morphologically, the Lemnaceae is among the best defined groups in the
angiosperms, and we are continuing to recognize it at the family level. Within the Lemnaceae it
is thought that evolution has proceeded from complex ancestors (e.g., Spirodela and Lemna) to
reduced taxa such as Wolffiella and Wolffia, and it is possible that a number of currently recognized
genera (e.g., Lemna, Spirodela, and Wolffiella) are paraphyletic (Les et al. 1997).
Because of their huge numbers and rapid reproduction, DUCKWEEDS can have pronounced
influences on aquatic habitats (Hicks 1937). They are important as food for waterfowl and fish,
are easily dispersed by various mammals and birds, and can become pests. Vegetative reproduction
is most important (flowering and fruiting are rare) and surprisingly efficient. According
to Landolt (1998), “Under optimal conditions, the number of fronds may double within 24h,
and one frond can easily develop into a population of over a million fronds within a month.”
Due to this prolific asexual reproduction, Lemnaceae are being studied by NASA as a possible
food source for prolonged space voyages. Also, because of their small size, rapid reproduction,
and ease of laboratory maintenance and manipulation (Jordan et al. 1996), duckweeds are potentially
valuable subjects for a variety of laboratory experiments. Lemnaceae have also been
used in wastewater treatment systems as biological filters, and the plants are sometimes harvested
to make a high-protein feed for livestock (Tyrl et al. 1994). Wolffia fronds are used in se
Asia as a vegetable, while Lemna gibba is used in Israel as a vegetable and salad (Landolt 2000).
If flowers are produced, the whole plant floating on the water appears to serve as a pollen
carrier (Cox 1988)—this type of hydrophily (= water-mediated pollination) occurring at the
water surface is known as epihydrophily (Philbrick 1993). Some workers (e.g., Cox 1988) further
divide epihydrophily into a category in which pollen is transported just above the surface of
the water (e.g., various Lemnaceae, Vallisneria—Hydrocharitaceae) and a category in which
pollen is transported directly on the surface of the water (e.g., Elodea—Hydrocharitaceae).
The family is neither well-collected nor well-studied in East TX. All East TX species are
found on lakes, ponds, swamps, marshes, or other standing water or stranded on mud. They are
prominent chiefly in late summer and fall. According to Landolt (2000), “Generally, most
Lemnaceae species have expanded during the last years because of the warming of the climate
and eutrophication of the waters.” This easily recognized family contains the smallest known
flowering plants, members of the genus Wolffia. While generally easy to identify to genus, specific
determination is often difficult, and definitive examination of some characters (e.g., vein
number) requires the preparation of microscope slides; easily observed characters were used
wherever possible in the following keys. Chromosome number is often quite variable within a
species; for example, Spirodela polyrhiza has 2n = 30, 38, 40, 50, 80 (Urbanska-Worytkiewicz
1980; Landolt 2000). The following treatment relies heavily on works by Landolt (1980, 1986,
2000). (subclass Arecidae—Cronquist; order Alismatales—APG II)
FAMILY RECOGNITION IN THE FIELD: very small or minute aquatic plants (larger plants only a few
mm in size) free-floating on or just below the water surface.