Scanning electron micrograph of spores of common horsetail, Equisetum arvense. The picture shows spores in a sporangium in the strobilus (cone) of a fertile frond. Each spore (green) has a hygroscopic elater (yellow), tightly adpressed to its surface.The elater acts like a coiled spring to assist spore dispersal. The spores here are damp, inside an immature cone. As the cone dries out, the spores separate, and the elaters uncoil. Still attached to the spore, they extend to become four long stems, each ending in a paddle (e.g as a pair below centre). In this state, the spores can be carried away by air currents. The elaters remain extended until a spore lands on favorably moist soil. They then coil up, and the spore, at rest, germinates. E. arvense is a common weed of moist places. With its perennial rhizome, and resistance to herbicides, it is unwelcome in gardens. Mag x 360 at 10x8