Horsetail

 This strange plant is the Dutch rush Equisetum hyemale which isn't a rush at all - it's a member of that strange division of the plant kingdom known as horsetails. We found it yesterday, growing in a ditch beside the disused railway line that now forms the Derwent Walk Country Park in Gateshead.

To some these flowerless stems must seem like the dullest plants on the planet but they are living fossils, descendants of giant ancestors that once formed a major component of the flora in the Carboniferous swamps, 300 million years ago. The Dutch rushes that we found yesterday grew to about 60 cm. tall but their ancestors grew to tree-sized proportions, some 30 metres tall, and are commonly found as fossils in coal measures. At that time the amphibians were the dominant form of land vertebrate life.



Compression fossils of horsetail stems, like these that we found on the beach at Dawdon on the Durham coast, are easily identifiable as they show the same pattern of grooves and ridges found in present-day horsetails. These stems were about ten times greater in diameter than the Dutch rush stems in the picture above, so if you were to scale up its height in direct proportion this particular fossil ancestor might have been roughly the height of a double-decker bus when it was alive.

This is the distinctive cone of Dutch rush, with that little spike at the top. When it's ripe in spring .....

.... each of those hexagonal sections will separate and elongate on a short stalk, .... like .....

...... these, which belong to the common field horsetail Equisetum arvense. The spores that are released from those yellow sporangia under the hexagons are unique to horsetails ....




.... because each has four long arms called elaters which are curled around the spore during development but are deployed when it dries out and is released, increasing the aerial buoyancy of the wind-dispersed spore.

Rock of Ages

The Durham coast isn’t a top destination for fossil hunters, who find richer pickings further south under the crumbling cliffs around Whitby, but occasionally interesting fossils do turn up on our beaches. I found one has week, on Blast beach just south of Seaham, which gave a tantalising glimpse of how the landscape might have looked three hundred million years ago.

Blast beach was formerly a site for dumping colliery waste but in recent years the magnificent Turning the Tide project has made great strides in restoring this coastline to its former, pre-industrial glory. There are still large boulders on the beach that were transported with colliery debris and it was when I split one of these that I found the remains of a plant that had been embedded in the rock for three hundred million years.   The compressed, jointed stems, now turned to stone, belonged to a giant horsetail.If you double-click for a larger image you can see that there are two reasonably intact compressed fossil stems here - one running at an 11 'o'clock to five o'clock angle and the other running eight o'clock to two o'clock.

Back in the Carboniferous, when the coal measures and the land mass that is now Britain was nearer the equator, these strange plants formed tropical, forests in swamps that would have been home to early amphibians and giant dragonflies.

Horsestails still exist today, as 'living fossils', but are a pale shadow of their extinct ancestors. Gardeners are familiar with the field horsetail, which is a troublesome weed but only grows about a foot tall. Today specimens of the great horsetail, the largest horsetail species that grows in damp places along this coast, reach a height of about five feet. Judging by the width of the fragment of fossil stem that I found, this long-extinct plant must have been at least four times as tall. Larger fossils that have been found indicate that these ancient plants sometimes grew to a height of sixty feet or more. 

It's bad news when these appear in your garden. This is the spore-producing shoot of field horsetail Equisetum arvense, about to liberate thousands of spores which have a unique adaptation to aerial invasion, which you can see by clicking here.

Once it's established this plant is well-nigh impossible to eradicate. Its underground stems (known as 'devil's guts') run deep and fragment easily, regenerating new plants just when you think you've dig it all out. Still, it could be worse; back in the  Carboniferous, 300 million years ago, when the coal measures were laid down, these grew 10 metres tall. 

300 Million year-old Survivors

These are the fertile shoots of great horsetail Equisetum telmateia, photographed along the Long Walk at Howick on the Northumberland coast last weekend. Each one of those small polygon-shaped objects in the cone-shaped head releases hundreds of spores. You can see some micrographs of spores from a similar field horsetail here. Horsetails are the last living descendants of a group of plants that flourished back in the steamy Carboniferous swamps, 300 million years ago, when the coal measures were laid down. Carboniferous horsetails, classified in the genus Calamites, were essential similar to their living descendants but much larger, some reaching 10 metres in height. As soon as the fertile shoots of great horsetail have shed their spores they wither away, but by then the green vegetative shoots of this year's plants will have begun to elongate and will eventually form dense one metre-tall thickets of stems - minute compared with their extinct ancestors, but still impressive.

Living fossils

I found these strange, spore-producing shoots of field horsetail Equisetum arvense on a grassy bank near Durham city this afternoon. Known as the 'devil’s guts' to gardeners, on account of its devilish creeping underground rhizomes and resistance to herbicides that together make it almost impossible to eradicate, it produces this yellowish spore stalk first and green shoots only after these have shed their spores and withered away. Field horsetail grows to a height of around 30cm., but back in the Carboniferous, about 300 million years ago, plants that were 10 metres tall but otherwise identical flourished in steamy primaeval swarms. Like present-day horsetails, these gigantic ancestors’ stems were rich in silica and so were slow to decay, leaving excellent fossils whose basic features are almost identical to those of their present-day descendants. When I tapped the sporangia pictured here they release clouds of spores that drifted away on the wind......and if you want to see what these remarkable spores look like visit this post on my other blog, Beyond The Human Eye