Ferns reproduce by an alternation of generations (see reproduction), the fern itself being the sporophyte, which produces asexual spores. In most ferns the sporangia (spore-bearing sacs) are borne in clusters (called sori), which appear as brown dots or streaks on the underside of the leaves. Although no present-day ferns reproduce by seeds, there are fossils of some fernlike plants that were seed-producing, and it is believed that the seed plants (e.g., the gymnosperms and true flowering plants) evolved from fernlike ancestors.
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