Horsetail plants have a very ancient lineage that extends back some 200 million years. Similar to other primitive vascular plants, they exhibit an asexual reproductive structure, the sphenophyte strobilus, more commonly known as a cone. Unlike the better-known pinecone, however, the strobili of the horsetail plant may only contain one type of spore since they are homosporous. Horsetail cones, each of which encases many sporophylls within a single sporangium or case, form along the uppermost region of the plant’s branches. When the sporophylls are dispersed by the wind, they germinate into small, free-living gametophytes that may produce either male or female reproductive organs. If these gametophytes are exposed to enough moisture, the sperm cells are released from the male organs and swim through the water to fertilize female eggs, beginning the next phase of the plant’s life cycle.