David Beeson, September 2022
Evolution is powerful. If you fail to fit in, something else will take your place, and freshwater is today only filled with the fittest of plants. Yet, those plants originated as marine organisms that migrated onto the then uncolonized land. Here the conditions were very different, and evolution forced them to support themselves, to cope with a drying atmosphere and obtain carbon dioxide, water, minerals needed for their biochemistry and sunlight energy. It took a long time … with steps via the liverworts, mosses, horsetails and ferns before the dominant flowering plants emerged.
Somewhere along this journey some species changed direction and moved back into water – fresh water. For, we are told, they did not arrive directly from a marine environment.
So, what makes freshwater Angiosperms so special?
These hydromorphs come in all shapes and sizes but have one feature in common – their roots and stems, and possibly other structures, are full of holes. A tissue type called aerenchyma. Air spaces are supported by the thinnest of internal ribs of small, water-filled cells. These gaps allow easier gas flows from the aerial parts down to the oxygen-needing roots, which sit in oxygen-deficient, anaerobic mud. The air provides buoyancy, so keeping the photosynthetic parts nearer the light.
A trick to show this aerenchyma is easy. Collect some aquatic plant, cut its flimsy stem at an angle and weigh it upside-down in a jam jar. When exposed to light the photosynthetic oxygen moves around the stem and will emerge as bubbles. You could collect these gases and, using a glowing splint of wood, by plunging it into the gas it will relight – a sure sign of oxygen. A similar experiment with a land plant will show little or no emerging gas.
Some plants evolved to dwell on the water surface. Duckweeds are a good example. They appear to have only one or a few airy, floating, leaves and trailing, fine roots. However, the most determined of you, in June or July, will collect some and see minute flowers. (Now, there is a challenge for you!)
What to do if you are totally emerged? There are no bees available to transport the pollen to a stigma. So, the Hornwort (Ceratophyllum demersum) bears small underwater flowers whose anthers detach and float to the surface, they burst open (dehisce) releasing heavy pollen that sinks down to pollinate receptive stigmas. Clever! The Canadian Pondweed, an introduced plant that is only female, has long, fine flower stems to thrust its flowers, in late summer, onto the water’s surface.
Plants rooted in the mud, yet with emergent leaves can suffer badly in a cold winter. The Water Soldier (Stratiotes aloides) lays down heavy calcium deposits in autumn, increasing its density – so it sinks to a less-stressful environment. It reverses the process in spring and again floats – an then produces its evil-smelling flowers that are fly pollinated.
Marestail, Hippuris vulgaris, is an emergent plant that many think is a horsetail. (Not so, as small green flowers can be spotted by those brave enough to wade out to view them in summer.) Here, the plant’s internal structures are quite different in the aerial and aquatic parts. Long, thin, transparent, underwater leaves are replaced by short, thick and stiff ones with both a thick, waterproof, cuticle and stomata. The plant is a great example of the requirements for both environments.
We have lilies in our eco-pond. Some are rooted in the mud, others free-floating with delicious small yellow flowers. Their ‘Big Brother’ lives in the Amazon with leaves 2m across and covered in vicious spines to attempt to eliminate consumption by fish. Our version is much tamer but with stomata only on its upper leaf surfaces.
When in Central East Africa I encountered Water Hyacinth. This, like Canadian Pondweed, is an introduced exotic that is toxic to local wildlife and has seized the opportunity to cover vast tracts of water. It blocks waterways, shades out underwater plants, increases transpiration and generates an environment that encourages luxuriant mosquito populations and plentiful bilharzia-carrying snails. Be careful what you put in your eco-pond … my Canadian Pondweed is a menace but controllable.
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