Azolla

Mosquito fern, fairy moss

Azolla is the world's smallest fern as well as the most significant economically. Occupying a unique botanical niche as one of only a few water ferns, it produces clusters of 100 to 200 overlapping 1/32-in. (0.8-mm) scalelike leaves that float like duckweed on tranquil ponds and margins of quiet creeks, sometimes settling on their muddy perimeters. In favorable habitats it can reproduce its biomass in as little as three days, sometimes forming solid mats of foliage so dense that it supports the weight of tree frogs. It enjoys a reputation, reflected in its common name, of preventing mosquitoes from breeding. Although rarely fertile, the genus is unusual in having two types of spores—female megaspores and male microspores—

Azolla in a container at Elandan Gardens.

Azolla in Connor Creek on the Pacific coast near Copalis, Washington. This remarkable little fern is credited (Stoll 2006) with reversing a global warming trend some 55 million years ago at which time, a scientific arctic coring expedition reveals, the North Pole was a balmy 74°F (24°C). With its naturally rapid growth, Azolla absorbed massive amounts of carbon dioxide and consequently slowly cooled the earth's climate.

known botanically as being heterosporous. The seven species, distributed worldwide, are separated with difficulty and require the close inspection of a scanning electron microscope. The genus name is derived from the Greek azo, to dry, and olluo, to kill, acknowledging the fact that these ferns will die if allowed to dry out.

In western cultures Azolla is frequently used as an ornamental curiosity in terrariums or water features in Zones 6 to 10. (Hobbyists should be aware, however, that chemical fertilizers and insecticides are reputed to destroy the growth.) Cold weather and high light turn the foliage into a striking, autumnal rosy red, but severe cold will destroy the floating frond material. Nature will persevere, however, as buds dropped to an underlying layer of organic matter will resume growth in springtime. In addition to decoration, the genus serves functionally and variously as food for fish (for which it was even used in Biosphere II), livestock, and waterfowl, and the tortoises in the Galapagos eat it like candy (Gottlieb, pers. comm.).

In the rice-producing areas of Asia, especially China and Vietnam, Azolla is a vital crop for sustaining the productivity of paddies. With its symbiotic relationship with Anabaena azollae, which fixes nitrogen in Azolla, it has increased yields by over 150 percent as well as significantly improving the protein content usually lacking in rice. Paddies are inoculated with Azolla, then drained, leaving the enriched Azolla to decompose as a green manure. The application is so critical to cultivation that in centuries dating back to the Ming Dynasty only a few small villages, with the fortunate inhabitants possessing insider knowledge passed from generation to generation, monopolized propagation. Sons were not provided with the traditional secrets until demonstrating that they would continue living in the privileged village. And daughters were not given the information at all, lest they marry outside the village and carry the precious information away with them.

Not surprisingly, Azolla receives an extraordinary amount of scientific attention. Today more than 600 strains of Azolla, "little fertilizer factories" (Moran 2004), are under study for insect and fungal resistance as well as improved cold and heat tolerance.

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