Horsetail spores. Coloured scanning electron micrograph (SEM) of spores of a horsetail (Equisetum arvense) plant. The spores (round) bear elaters (brown), which expand and contract with changes in humidity. Elaters help to tangle spores together into a clump called a propagule, and to dig the spores into the soil.
Credit: SUSUMU NISHINAGA/SCIENCE PHOTO LIBRARY