Like ferns, horsetails (Equisetum) do not produce seeds. They produce spores instead. The spores are produced in a beautiful structure called the strobilus. These small spores (around 0.05 mm) have evolved an amazing adaptation that increases their dispersal and therefore makes it easier for new horsetail plants to expand further away.
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The Equisetum spores have four appendages (elaters) that quickly deploy or retract depending on the level of environmental humidity (the movement of the spores in the video is played in real time).
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Understanding the biomechanics of the walking and jumping of these spores could one day lead to the engineering of self-propelled objects.
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Equisetum plants (horsetails) are living fossils! In other words, Equisetum is the only remaining representative genus of a group of plants that are close relatives of ferns. Millions of years ago, horsetails were very abundant on Earth.
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During the Carboniferous period, these plants were huge (giant trees in some cases) and very abundant. Due to geological and environmental factors, the incredibly abundant dead organic matter generated by horsetails (and other plants) did not decompose. Over millions of years, pressure and heat made all this plant organic matter fossilize in the form of coal (this is why coal is a “fossil fuel”).
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So basically, every piece of coal that gets burned likely contains some fossilized horsetail dead organic matter.
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For this video I used an Olympus CX31 microscope and up to 400x magnification.
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For more information about the movement of Equisetum spores:
Philippe Marmottant, Alexandre Ponomarenko, Diane Bienaimé. The walk and jump of Equisetum spores. Proceedings of the Royal Society B: Biological Sciences, Royal Society, The, 2013, 280 (1770), pp.20131465. ffhal-00911670

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