Field horsetail (Equisetum arvense) stems and cones. Fertile stems bear a structure called the cone or strobilus. These cones release the spores (containing the male reproductive cells), and are part of the sexual reproduction of this plant. Horsetails normally use asexual reproduction using rhizomes (underground stems). 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).