Azolla a heterosporous leptosporangiate floating fern of order Salviniales, family Azollaceae. It has two sections, Euazolla (trichomes occurring on leaves only) and Rhizosperma (trichomes occurring on leaves and stem). Azolla is fairly common on the surface of quiet freshwaters of roadside ditches, ponds, fallow rice-fields, etc of both tropical and temperate regions. The plant consists of roots, stems (rhizomes) and leaves. Azolla pinnata (occurs also in Bangladesh) is 1.0-1.5 cm long and usually triangular in appearance. The leaves are nearly quadrangular, about 1mm long and 0.5 mm wide. They arise from the dorsal surface of the stem and are arranged in two alternate rows. Each leaf is deeply bilobed, the dorsal lobe is thick and photosynthetic while the ventral lobe is thin and colourless or may be pink due to anthocyanin. From an axil of each leaf lateral branches develop. Several adventitious roots with root hairs develop from the under surface of the stem or branches. The dorsal lobe of each leaf has cavities which harbour symbionts, such as Anabaena azollae (a blue-green alga, also called Cyanobacteria) and a few genera of Eubacteria, predominantly Azotobacter species. The Anabaena azollae is heterocystous and can fix atmospheric nitrogen like that of legume bacteria by the enzyme nitrogenase.

Azolla pinnata (red form); A. pinnata (green form); Anabaena azollae a symbiont; Enlarged surfaceview of A. pinnata; Enlarged ventral view of A. pinnata

Azolla can reproduce vegetatively or by sexual means. In vegetative reproduction, the oldest lateral branch at the base of the stem is fragmented and gives rise to an independent plant. Under optimal conditions the doubling time could be about 3 days. The plant is sporophytic and alternates with a small gametophytic phase. The plant produces two types of sporocarps, the small megasporocarp (female) and the large microsporocarp (male) on the axil of the oldest (first) leaf of a lateral branch during winter and early spring in the tropics. Photoperiod and temperature appear to induce sporulation. The megasporocarps and microsporocarps always grow side by side in a pair. The plant can be cultivated on a large scale throughout the year. Well-illuminated pond water about 100 cm deep is ideal. Optimum yield (1.0-1.5 m ton fresh Azolla/ha/day) can be achieved applying a solution of 10 kg urea and 5 kg Muriate of Potash/ha/day and harvesting every day. Foliar spray of the fertilisers however, is more productive than adding a solution in the water.

Azolla has a variety of uses. The International Network in Soil Fertility and Fertiliser Evaluation for Rice indicated that one crop of Azolla (about 15 m ton/ha) incorporated before or after transplanting rice increased yield equivalent to that obtained due to 30 kg urea-N/ha. The benefit of two crops of Azolla incorporated before or after transplanting rice, was equivalent to the application of 60 kg urea-N/ha. Azolla may also be dried or composted and applied as biofertiliser. Azolla compost appeared to be very effective in rice production. In addition to nitrogen, it provides other nutrient elements to the soil, improves soil structure, and reduces water evaporation and weed incidence when grown as dual crop. There is evidence that Azolla is good as feed for fish, poultry, and livestock because of its balanced amino acids, 20.4-31.4% crude protein, 4.1-5.8% crude fat and 11-21% crude fibre. Adding 2% Azolla granules with normal feed resulted increased egg production in poultry by about 4%. A strain of A. filiculoides has been found to be a cheap indicator of arsenic pollution in groundwater. The plant can also be used as a wastewater treatment agent and in biogas production. [Abdul Aziz]

See also aquatic plant.