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(horsetails), a family of plants of the class Equisetopsida. The stems have lost the capacity for secondary growth in thickness, and the leaves are reduced. The strobiles (spikes), which bear sporophylls with sporangia, are located on the shoot apexes. Archegonia and antheridia form on the bisexual or unisexual gametophytes (prothallia) that develop from the spores. Spermatozoids with numerous flagella form in the antheridia. After fertilization, the gametophyte gives rise to one or more sporophytes; thus, there is regular alternation of sexual and asexual generations. Representatives of this family include extant species of the genus Equisetum and extinct species of the genera Equisetites, Schizoneura, and a few others.