Nature Notes by Dr. Frank Lang
Have you ever noticed that small ponds, the edges of slow streams and backwaters in southern Oregon sometimes turn brick red: Its enough to make a pharaoh faint.
The cause is not an act of retribution, but a tiny, floating fern, Azolla, also known as the mosquito fern, that forms red anthocyanin pigments when under stress or too much sunlight. We think the red pigment protects the photosynthetic apparatus from solar overload.
Mosquito fern shares the same habitat as the equally small flowering plant, duckweed. The chartreuse duckweeds have small oval leaves attached to tiny stems. Roots may or may not be present depending on the genus. The flowers of the duckweed are appropriately small, with tiny stamens and a tiny pistil, as individual flowers, on a single plant. No sepals or petals, just the reproductive parts.
The mosquito fern does other amazing things besides turn red. The lower surface of the upper leaf has pouch-like cavities which contain a blue-green alga. These two organisms have a symbiotic relationship where both partners benefit. The alga changes atmospheric nitrogen to a form that can be used by other plants, especially mosquito ferns. In return, Azolla supplies the alga with some nutrients and a protected cavity at the surface of the water in the sunlight.
Enough nitrogen is produced to share, however. The water fern can be used as a green manure in cultivating rice – a far less expensive nitrogen fertilizer than the usual energy-intensive sources.
The mosquito fern, six species world-wide, has tiny overlapping leaves along a slightly elongated stem with small roots that hang below the water’s surface. Its dangling roots and those of rooted duckweeds make the plants popular with those of us who raise live-bearing tropical fish like guppies. The dangling roots make places for young fish to hide from hungry parents, uncles, aunts and cousins.
We think the mosquito fern gets its common name because the plants can grow so densely on the water’s surface that mosquitoes cannot penetrate the mass of fronds to lay their eggs nor can their larvae reach the air to breath.
To find water ferns in the Rogue Valley try Railroad Slough north of Tolo just before you reach Gold Ray Dam, or the farm pond just before you reach Crowson Road as you head south of Ashland on Old US 99.
— Dr. Frank Lang