Water Horsetail

The Equisetum genus, which includes water horsetail(1), is a relict of spore producing ancestors that grew in the Carboniferous period (300 million years ago). These ancient plants formed much of the world’s coal deposits; some grew up to 30m tall(2).

You can see this carbonised fossil, from nearby,
at the Museum in the Park, Stroud

Like those by the QEII pond and along the Stroudwater Navigation, the fossilised specimen looks like a bottle brush, though new spring shoots are naked, and look quite similar to asparagus.

Once around 10 cm high, they develop needle-like green branches, which are arranged in whorls at nodes up the stem. The purple-toothed sheaths, above each node, are the non-photosynthetic leaves.

While by late spring only non-fertile shoots emerge, in early spring the first  stems are tipped with a cone, the strobilus, which ripens to release millions of green spores. Each spore has four short legs which may help gliding in the air, and, intriguingly, once on the ground, allow it to walk and jump as the humidity levels change(3), to aid its dispersal.

The segments between the nodes gradually decrease in length up a stem in a regular way, which is said to have inspired the 17th century Scottish mathematician John Napier to invent logarithms(4).

 

Early morning dew on the plant forms as almost spherical droplets. Various plants, such as grasses, elicit this same superhydrophobic effect(5).

Horsetails have evolved a unique way of achieving this, producing a micro-bobbly surface by incorporating silica balls under a waxy coating.

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magnified branch

magnified stem

If you rub a branch it feels like very fine sandpaper, and traditionally a wodge of the plant was used to polish metal, such as pewter, and wood, which gave rise to the folk name ‘scouring rush’.

Links

1 The Wildlife Trusts (Beds, Cams, Northants): a revised key to the horsetails

2 In defense of plants: Ancient Equisetum

3 The walk and Jump of Equisetum spores

4 Maths is fun: introduction to logarithms 

5 Maths Inside: Nature’s Raincoats