Small, floating herbs, often forming a mat over still waters, rhizome fins, horizontal, branching alternately to the left and right, bearing small, close leaves and fine, simple roots, protostelic. Leaves +/- imbricate, alternate, bilobed, papillose. Sporangia borne in sporocarps, in the axils of the submerged lobes of the leaves, enclosed in a thin indusium, the sporocarps of 2 types, microsporocarp large, with several microsporangia each with many microspores, megasporocarp with a single megasporangium with an single megaspore; spores globose, trilete.
A mongeneric family with about 6 species, found throughout the tropical and temperate regions of the world. Azolla pinnata is the only species in Papuasia.
Small, free-floating aquatic ferns, capable of rapid vegetative reproduction by fragmentation and forming a carpet on still water. Rhizome horizontal, zig-zag, freely branching alternately to the left and right, bearing close, minute leaves and spaced simple roots, vascular system poorly developed, a medullated protostele. Leaves imbricate, alternate, deeply bilobed; the aerial lobe overlapping the rhizome, thick, papillose, with a basal cavity containing a symbiotic filamentous blue-green algae (Anabaena azollae); the submerged lobe thin-membranous, not papillose. Sporangia borne in sporocarps, usually paird micro- and megasporocarps, borne in the axils of the submerged lobes, basally on the branches, quite enclosed by a thin indusium; microsporocarp large, gobose, containing several to many globose microsporangia, each containing 32 - 64 microspores; megasporocarp smaller, containing a single megasporangium with a single megaspore. Spores globose, trilete, smooth to variously pitted or sculptured; microspores imbedded in the outer edge of several mucilaginous masses (massulae) in the microsporangium, the massulae bearing several to many, hooked (glochidiate) or non-hooked, septate or non-septate processs on one or all sides; megaspores with 3 or 9 apical massulae or 'floats'.
Distribution: A more or less cosmopolitan genus of about 6 species, with a single species in Papuasia:
Croft, J.R. 1985. Ferns and Fern Allies, in Leach, G.J. & Osborne, P.L. 1985. Freshwater Plants of Papua New Guinea. 33 - 74, f. 6 - 13, pl. 5 - 7.
Follieri, M. 1977. Classification and phyllogeny of living and fossil water ferns of the genus "Azolla". Webbia 31: 97 - 104. F. pl. 1 - 2.
Johns, R.J. 1981 The ferns and fern-allies of Papua New Guinea. Part 6 - 12. P.N.G. Univ. Tech. Res. Rep. R 48 - 81. (incl. Azollaceae)
Konar, R.N. & Kapoor, R.J. 1972. Anatomical studies on Azolla pinnata. Phytomorphology 22: 211 - 223, f. 1 - 5.
Konar, R.N. & Kapoor, R.J. 1974. Embryology of Azolla pinnata. Phytomorphology 24: 228 - 261, f. 1- 19.
Lumpkin, T.A. & Plucknett, D.L. 1980. Azolla: botany, physiology, and use as a green manure. Econ. Bot. 34: 111 - 153, f. 1 - 20.
Moore, A.W. 1969. Azolla: biology and agronomic significance. Bot. Rev. 34: 17 - 34.
Reed, C.F. 1954. Index Marsileales et Salviniales. Bol. Soc. Brot. 2 ser. 5 - 61.
Reed, C.F. 1965. Distribution of Salvinia and Azolla in South America and Africa in connection with studies for control by insects. Phytologia 12: 121 - 130.
Shen, E. Yu-Feng 1961. Concerning Azolla imbricata. Amer. Fern J. 51: 151 - 155, pl. 10.
Sweet, A. & Hills, L.V. 1971. A study of Azolla pinnata R. Brown. Amer. Fern J. 71: 1 - 13, pl. 1 - 4.
A single species in Papuasia:
Synonyms: Azolla imbricata (Forst.) Nakai, Azolla pinnata var. imbicata (Forst.) Bonap.
Plants small, 1.5 - 2.5 cm long, with a +/- straight main axis with pinnately arranged side branches, progressively longer towards the base, thus roughly triangular in shape, the basal branches themselves becoming pinnate and eventually fragmenting as the main axis decomposes to form new plants. Roots with fine lateral rootlets, having a feathery appearance in the water. Leaves minute, 1 -2 mm long, overlapping in 2 ranks, upper lobe green, brown green or reddish, lower lobe translucent brown; minute, short, plae, +/- cylindrical unicellular hairs often present on the upper lobes. When fertile, round sporocarps 1 - 1.5 mm wide can be seen on the under side at the bases of the side branches.
Habitat: Floating on the surface of small, still ponds or back waters with not wave action, at low to middle altitudes. It becomes especially abundant in water with high nutrient levels, such as ponds in cattle paddocks, where it can completely cover the water surface. It is quite possible that Azolla was introduced to New Guinea with cattle. In Papuasia the altitudinal distribution falls into two distjunct ranges; lowlands populations 3 - 60 m altitude and highlands populations 1000 - 3000 m altitude. However, there is no obvious difference between plants from the highlands and those from the lowlands.
Distribution: Africa and Madagascar, India, south east Asia, China and Japan, Malaya and the Philippines, the New Guinea mainland and Australia. Azolla do4es not seem to have been collected from the Bismarck Archipelago nor from the Solmon Islands. The paucity of specimens from Irian Jaya probably reflects the low frequency of collecting in that area.
Notes: The leaves often have a maroon-red tinge and the water can appear to be covered by red velvet from the distance. The upper surface of the leaves are totally water-repellant, and if completely submerged the plants quickly refloat with the right side up (see also Salvinia).
Sweet & Hills (1971) consider Azolla imbricata to be a distinct and recognizable variety of Azolla pinnata; they cite Darbyshire 803 as being a specimen intermediate between the two varieties. Inspection of that specimen in LAE shows that it is not significantly different from the other specimens from Papuasia.
In southeast Asian countries, it is especially common in (wet) rice-fields. It is used as a natural fertiliser, taking advantage of the nitrogen fixing abilities of the symbiotic blue-green algae (Moore 1969, Lumpkin & Plucknett 1980).