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.