Tuesday, 14 April 2009

Horsetail Cones

It’s just possible that you may have noticed the weird looking things shown in the photographs above and wondered what they were (?) They are, in fact, the fertile, spore-producing fronds of curious plants called ‘Horsetails’ and are sometimes referred to as ‘Horsetail cones’. These fertile fronds emerge in April and are soon replaced by the more familiar sterile fronds - which consist of green, cylindrical, jointed stems around which are arranged whorls of horizontal branches.
The cones on the fertile fronds consist of tightly packed hexagonal plates, each of which is attached, via a short stalk, to the central stem. Eventually the central stem elongates, forcing the plates apart. On the underside of each plate are white capsules (‘sporangia’) containing the green spores (the sporangia are plainly visible in the top photograph). When the sporangia are exposed to air they dry out and split. Air currents then blow through the separated plates, releasing the spores into the environment. Each tiny spore is equipped with four strap-like structures called ‘elators’. In dry weather the straps are completely unfurled, allowing the spore to float and remain airborne. When the ambient relative humidity rises, as in wet weather, the elators wrap themselves around the spore and it drops to the ground where, hopefully, it can find a suitable spot to germinate in.
The top photograph shows the fertile fronds of Field Horsetail (Equisetum arvense) whilst the bottom photograph shows a fertile frond of Great Horsetail (E. telmateia). E. arvense is very common on the sandy soils of the Mersey valley – and there is no doubt that many allotment holders and gardeners consider it to be a pernicious weed. It forms extensive colonies, based on a system of branching, underground rhizomes which can be very difficult to eradicate. E. telmateia is much scarcer and so far I have only found it in two sites – although recently I was shown a photograph which leads me to believe that there may be a third.
So far I have found two other species of Horsetail in the Mersey Valley: Marsh Horsetail (E. palustre) and Water Horsetail (E. fluviatile). These tend to grow in water (although E. palustre can grow on dryer ground) but both produce their fertile fronds at the same time as their sterile fronds.
Horsetails are often referred to as ‘fern allies’ but recent research (see ref. 2) has shown that they are, in fact, true ferns. Other, so-called fern allies, such as Club Mosses and Quillworts, have been shown to be less closely related to ferns than ferns themselves are to flowering plants. Nevertheless, Horsetails appear to have an ancient lineage. They are believed to be closely related to extinct plants called Calamites which flourished during the Carboniferous period 345 to 280 million years ago. They had, like Horsetails, hollow, jointed stems, but those stems were thickened allowing the Calamites to grow up to 20 m tall. The fossilised remains of Calamites and other plants gave rise to today’s coal measures.

Dave Bishop, April 2009


1. ‘The Illustrated Field Guide to Ferns and Allied Plants of the British Isles’ by Clive Jermy and Josephine Camus, Natural History Museum Publications, 1991.

2. ‘A Natural History of Ferns’ by Robbin C. Moran, Timber Press, 2004.

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