Free Floating Plants  

 
Species: Azolla mexicana C. Presl, Mexican water-fern
Family: Azollaceae

Water-ferns are small, flat, compact, moss-like plants that float freely on the water surface, often forming extensive reddish mats. Young plants are generally bright green, becoming pink, red, or dark brown as the plant matures and when exposed to strong sunlight. Upon close examination, these floating mats are seen to consist of many tiny ferns with multiple pairs of individual overlapping scale-like leaves that resemble a cedar leaf. Each plant may have a branching stem with several pairs of leaves, but only a single dangling root.

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Leaf: The leaves are small (1 mm) and overlapping with each leaf consisting of 2 unequal lobes. The larger lobe is submerged and serves to keep the plant afloat. The smaller lobe remains above the water, and contains chambers of blue-green algae (cyanobacteria).

Stem: The inconspicuous, branched, stem is 1-2 cm long and generally hidden by the overlapping leaves.

Flower: None produced. 

Fruit: No fruit produced; instead reproduction occurs by spores located in a chamber on the lower submerged leaf-lobe.

Root: A single root is on the underside of the plant.

Propagation: Spores and fragmentation of individual plants.

Importance of plant: Blue-green algae in the leaves can absorb (fix) nitrogen from the air, something plants cannot do. This provides nutrients for the plant, while the plant provides shelter and minerals for the blue-green algae. Water-fern is used as green fertilizer in rice paddies because of its nitrogen fixing ability. It also provides food for waterfowl and cover for small invertebrates. Water-fern is considered a threatened species in British Columbia.

Distribution: Western North America and northern South America. Other similar species of water-fern are found nearly worldwide.

Habitat: Sheltered water. Occasionally found stranded on wet soil when water levels drop.

May be confused with: Other small floating plants including duckweed (Lemna sp., Spirodella sp.) or water meal (Wolffia spp.). These plants are brighter green, do not become red when exposed to full sun, and donít have the compact overlapping leaf arrangement of Mexican water-fern. Duckweed fern (Azolla filiculoides), which is less common and slightly larger, may also be found in our area. A compound microscope is required to positively distinguish species of Azolla.

Photographs: Closeup of Azolla mexicana

Line Drawings: Azolla mexicana


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