Calamites was a the giant horsetail of the Carboniferous Period, 300 million years ago. Whorls of narrow leaves grew from each joint in the stem. In the close up you can see what appears to be a joint at the top of the picture.
Along with giant club-mosses, the horsetails of the lush tropical forests of the Carboniferous were fossilised to form coal seams. In West Yorkshire we still derive most of our electricity from coal-fired power stations so this entry in my online nature diary has, most likely, been powered in part by solar energy stored by these giant horsetails 300 million years ago.