Botany picture #43: Azolla pinnata

Today we went to the parks around Lake Ginninderra in Belconnen. Very nice playground, lakes with many water-birds, a creek with impressive rocks along it, so all in all we made good use of the fine weather. The plants floating on the small lake in the park seen above are the topic of today's botany picture post.

Several lineages of land plants have independently evolved representatives that are adapted to floating on the surface of lakes or slow-moving rivers. The best known are duckweeds which are flowering plants in the Araceae family even if one would not know it from looking at them; their morphology is drastically reduced to a small flat or bulbous thallus, and in two of the five genera even the roots have been lost. Another lineage that has produced a similar floating representative are the complex thalloid liverworts, with Ricciocarpus natans. The photo above now shows a member of a third lineage to independently evolve this habit, the ferns. There are two small genera of free-floating ferns, Azolla and Salvinia, and this is Azolla pinnata (Salviniaceae).

These water-ferns are exceptional among ferns in that they are heterosporous. While most other ferns have only one kind of unicellular spores, the water-ferns produce many small male and fewer large female spores, and the development of the prothallium (gametophyte) takes place inside the spore. That is the first step towards the evolution of true seeds, and the seed plants must have gone through the same stage in their evolution hundreds of millions of years ago. Another interesting thing about Azolla specifically is that it forms a symbiosis with a cyanobacterium that fixes atmospheric nitrogen for its host so that Azolla can grow in more nutrient-poor water than most other floating plants.

Perhaps the most remarkable thing about Azolla is the "Azolla event": congeners of the plants pictured above are hypothesized to have played a crucial role in carbon sequestration 49 million years ago.