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Characters of the family.


Almost cosmopolitan, throughout wet tropics and warm temperate regions: present Asia-Tropical:
Almost cosmopolitan, throughout wet tropics and warm temperate regions (about 6 or 7 species); throughout Malesia (1 species).


Azolla maintains a symbiotic association with the cyanobacterium (blue-green alga) Anabaena azollae Strasb., which is able to fix atmospheric nitrogen; this allows the association to grow in nitrate-poor environments that cannot easily be colonised by other hydrophytes (Moore 1969; Lumpkin & Plucknett 1980, 1982; Shi & Hall 1988). (277)

Reproduction in Azolla is often vegetative, by fragmentation of the stem; this enables rapid population growth under favourable environmental conditions, and consequently Azolla can often become an aggressive weed, clogging waterways and drainage systems. Azolla also possesses an advanced heterosporous life cycle, however, with distinct micro- and megaspores. The gametophytes are endosporic (retained within the spore), and are consequently protected from desiccation of the environment. If the megagametophyte has been fertilised, it can therefore survive periods of seasonal drying and allow continuity of the population.


Extant species of Azolla have historically been classified into two sections, Azolla (4 or 5 species) and Rhizosperma (Mey.) Mett. (2 species) (Mettenius 1847), although these taxa have also been regarded as subgenera (Strasburger, 1873). A phylogenetically more acceptable supraspecific classification of the genus has recently been proposed by Saunders & Fowler (1993), however, as follows:
  • Subgenus Azolla
    • sect. Azolla (4 or 5 species)
    • sect. Rhizosperma (1 species, also in Malesia)
  • Subgenus Tetrasporocarpia Saunders et Fowler (1 species).


The chromosomes of Azolla are the smallest recorded for pteridophytes (Loyal 1975), and this has often resulted in the publication of inaccurate chromosome counts, reviewed in Stergianou & Fowler (1990). All Azolla species have a diploid chromosome number of 44, except A. nilotica Decne. ex Mett. which is 2n = 52 (Stergianou & Fowler 1989, 1990). Triploids (2n = 66) have been discovered in four species (Stergianou & K. Fowler, l.c.), and one tetraploid (2n = 88) population has also been reported (Stergianou & Fowler, l.c.; Saunders & Fowler 1993).


As a result of the nitrogen fixing capability of the Anabaena endosymbiont, Azolla is useful as an organic fertilizer is tropical lowland rice cultivation and has been used in Chinese and Vietnamese agriculture for over two thousand years (Moore 1969; Lumpkin & Plucknett 1980, 1982). Research and effective management practices have increased the potential for higher rice yields when grown with Azolla. The average nitrogen fixing activity of Azolla is 1-2 kg N ha-1 day-1 (Watanabe 1982); this is sufficient to meet the nitrogen requirement of rice if the Azolla is grown for the period of one rice cropping. Liu (1979) has estimated that the effective use of Azolla in paddy fields can increase rice yields by an average 600-700 kg ha-1. Within Malesia, the use of Azolla in rice cultivation has mainly been restricted to the Philippines (Mabbayad 1987) where considerable agronomic research on Azolla has been conducted at the International Rice Research Institute (IRRI) at Los Baños. Recent attempts at hybridizing Azolla species (Do et al. 1989; Watanabe et al. 1993) have revealed positive heterosis in growth and nitrogen fixation abilities.

Azolla has also been grown with water bamboo (Zizania aquatica L.), arrow head (Sagittaria sagittifolia L.) and taro , and has been used in aquatic weed control and as a fodder for pigs, cattle, poultry and fish (Lumpkin & Plucknett 1982). Azolla is also proving to be an important antipollutant: its ability to extract phosphorus from eutrophic water, even after complete denitrification, has resulted in many investigations assessing its use as a decontaminant in sewage treatment (Shiomi & Kitoh 1987; De Wet et al. 1990). The formation of dense mats of Azolla on the surface of stagnant bodies of water has also led to an evaluation of its efficacy in mosquito control (Ansari & Sharma 1991; Rajendran & Reuben 1991).


A limited amount of research has been conducted using anthocyanins (Shimura & Terada 1967; Holst 1977; Ishikura 1982) although this was not considered in a taxonomic context, and only involved the identification of the compounds occurring. Related studies of the phenolic compounds in Azolla, including flavonoids, have been interpreted taxonomically (Van Hove et al. 1987). The chemical composition of the spore apparatus has also been studied (Toia et al. 1985; Van Bergen et al. 1993).

The phytochemical research that has proved most valuable for deducing taxonomic and phylogenetic relationships in Azolla has involved studies of isozyme variation (Zimmermann et al. 1989a, b, 1991a, 1994) and molecular data (Zimmerman et al. 1991b, 1993, 1994; Eskew et al. 1993; Van Coppenolle et al. 1993). Restriction fragment length polymorphism (RFLP) studies of the endosymbiont clearly indicate coevolution with the Azolla host (Van Coppenolle et al. 1995).


Saunders & Fowler - in Pl. Syst. Evol. 1993
Strasb., Über Azolla. 1873
Baker - in J. Bot. 1886: reprinted in Handb. Fern Allies (1887) 137
Reed - in Bol. Soc. Brot. 1954
Schneller - in Kubitzki (ed.), Fam. & Gen. Vasc. Pl. 1. 1990: 57
Mett. - in Kotschy & Peyritsch, Pl. Tinn. 1867