Horsetail spores. Coloured scanning electron micrograph (SEM) of spores of a horsetail (Equisetum sp.). The spores (orange) bear elaters (green), which expand and contract with changes in humidity. Elaters help to tangle spores together into a clump called a propagule, and to dig spores into the soil. Horsetails are the only living group of a primitive family of plants, the Sphenopsids, that date back to the Devonian period, 355-410 million years ago. Horsetails normally spread by asexual reproduction using rhizomes (underground stems). Magnification: x180 at 6x7cm size.