Horsetail strobili. Coloured scanning electron micrograph (SEM) of strobili from a horsetail (Equisetum sp.). Strobili are reproductive structures. They contain sporangiophores (brown), each of which holds 5 to 10 sporangia, which contain spores, the reproductive cells of the horsetail. Horsetails are primitive plants that have existed since the Carboniferous period (360- 299 million years ago). Today, although only 15 species still exist, they are found across much of the world. Magnification: x30 when printed at 10 centimetres wide.