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

338 AZOLLACEAE/AZOLLA
AZOLLACEAE Wettst.
AZOLLA, MOSQUITO FERN, OR WATER FERN FAMILY
AA cosmopolitan family of a single genus and only ca. 7 species (Lumpkin 1993) of floating
aquatics (sometimes stranded on mud) with highly reduced vegetative morphology (Saunders
& Fowler 1992). Because of their minute size and numerous leaves, the plants superficially resemble
mosses or liverworts. The family is often included in the Salviniaceae, but according to
Lumpkin (1993), the relationship is not close. Three families of heterosporous water ferns occur
in East TX (Azollaceae—Azolla, Marsileaceae—Marsilea, Pilularia, and Salviniaceae—Salvinia).
These groups are quite distinct morphologically, and traditionally the marsileaceous (Marsilea
and Pilularia) and salviniaceous (Azolla and Salvinia) lines were considered to have evolved
independently from different homosporous fern ancestors. However, based on recent morphological,
fossil, and molecular evidence, all three living heterosporous water fern families appear
to comprise a monophyletic group (Rothwell & Stockey 1994; Hasebe et al. 1995; Pryer 1999).
FAMILY RECOGNITION IN THE FIELD: tiny, moss- or liverwort-like, free-floating or mat-forming
plants that sometimes form conspicuous velvet-like, green to red mats on the surface of quiet waters.
REFERENCES: Schneller 1990a; Lumpkin 1993.
AZOLLA Lam. WATER FERN, MOSQUITO FERN, FAIRY-MOSS
Plants small, free-floating or mat-forming, superficially resembling some mosses or liverworts,
monoecious; stems prostrate; leaves minute, deeply bilobed, imbricate, deep green to reddish
(under stress); sporocarps in the leaf axils, of two kinds, the megasporocarps with 1 megasporangium
producing 1 megaspore, the microsporocarps with numerous microsporangia containing
numerous microspores.
AAzolla species are considered the world’s smallest ferns (Moran 1997). The upper emergent leaf
lobes are hollow and inhabited by a symbiotic nitrogen-fixing cyanobacterium (= blue-green bacterium),
Anabaena azollae Strasb., that combines, or “fixes” atmospheric nitrogen with hydrogen
to make ammonia, which can be utilized by the plants (Moore 1969; Shi & Hall 1988; Hoshizaki
& Moran 2001). Because of the resulting nitrogen content, Azolla species have been widely
used agriculturally as a fertilizer, and economically are considered the world’s most valuable
ferns. “Cultivating Azolla as an organic fertilizer for rice has been a centuries-old practice in
China and Vietnam …” (Moran 1997), dating back perhaps to the 11th century or even before.
Wagner (1997) reviewed the extensive literature on the uses of Azolla (e.g, biofertilizer, animal
feed, human food, medicine, water purifier). Because of these uses, particularly as a biofertilizer,
Azolla is considered by some to be the world’s most economically important fern (Moran 1997).
The common name, MOSQUITO FERN, is said to be “due to the belief that its dense covering on the
surfaces of ponds retards or prevents the growth of mosquitoes” (Nelson 2000). Their small size
and difficult to observe microscopic characters (e.g., megaspores), can make species of Azolla
extremely hard to identify (Hoshizaki & Moran 2001). The two species occurring in East TX are
quite similar, frequently confused, and of uncertain taxonomic status. Detailed study throughout
their entire ranges will be needed to determine if the two should continue to be recognized
at the specific level or lumped into a single variable species. The following key to species is
slightly modified from those in Lellinger (1985) and Lumpkin (1993). (Greek: azo, to dry, and
ollyo, to kill, alluding to death from drought, in reference to its dependence on water)
REFERENCES: Svenson 1944, Correll 1956, 1966a; Moore 1969; Lumpkin and Plucknett 1980; Perkins
et al. 1985; Moran 1997; Wagner (G.M.) 1997; Dickinson & Miller 1998.
1. Plants usually 0.5–1 cm long, dichotomously branched nearly throughout; megaspores not pitted,
densely covered with tangled filaments (high magnification required; also note: plants infrequently
fertile); leaves not or scarcely imbricate, the upper lobes usually 0.5–0.6 mm long _____ A. caroliniana