In February, Leonard and I were walking along Abbott’s Lagoon Trail in Point Reyes National Seashore (California) with friends, Chris and Edith. Passing a pond covered in green and red, I was tempted to ignore that aquatic plant. Thankfully Chris, a botanist, stopped and introduced me to a fascinating fern, red water fern (Azolla filiculoides).
Native to warm temperate and tropical regions of the Americas, red water fern is now naturalized in parts of every continent except Antarctica. A perennial, red water fern is not tolerant of cold, dying back in winter and surviving via submerged buds. Its habitat is ponds, small lakes, slow-moving streams and backwaters.
Red water fern, also called duckweed fern, floats in or on water and under favorable conditions can form mats up to 20 cm thick, completely covering a small body of water. The short, flat stems break into smaller segments as the plant grows. The scale-like leaves overlap one another along the stem. The leaves are usually green but due to changing temperature or light conditions can become reddish purple.
Nostoc azollae strain 0708 (formerly called Anabaena azollae strain 0708) is a filamentous blue-green algae (cyanobacterium) that lives in cavities formed by the leaves of red water fern. This extracellular symbiont can photosynthesize and fixes nitrogen. Red water fern provides Nostoc with fixed carbon while receiving combined new nitrogen from the algae.
Like other ferns, red water fern reproduces, under appropriate conditions, by spores contained in sporangia (spore sac). This member of the Salviniaceae Family is primarily dispersed by bits of plant material on the feet and feathers of birds, on the fur of mammals (otters, muskrats and others) on boats, on fishing equipment or on boots and other clothing. Red water fern is used as an ornamental in garden ponds or in aquaria. Carelessly discarded waste from these sources also spreads red water fern.
In my next post I will talk about the Jekyll/Hyde nature of red water fern, a plant that does not resemble our usual idea of a fern.