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Frond dimorphism refers to a difference inferns between the fertile and sterilefronds. Since ferns, unlikeflowering plants, bearspores on theleaf blade itself, this may affect the form of the frond itself. In some species of ferns, there is virtually no difference between the fertile and sterile fronds, such as in thegenusDryopteris, other than the mere presence of the sori, or fruit-dots, on the back of the fronds. Some other species, such asPolystichum acrostichoides (Christmas fern), or some ferns of the genusOsmunda, feature dimorphism on a portion of the frond only. Others, such as some species ofBlechnum andWoodwardia, have fertile fronds that are markedly taller than the sterile. Still others, such asOsmunda cinnamomea (Cinnamon fern), or plants of the familyOnocleaceae, have fertile fronds that are completely different from the sterile.
Only members of theOnocleaceae andBlechnaceae exhibit a propensity towards dimorphy, while no member of theAthyriaceae is strongly dimorphic, and only some representatives of theThelypteridaceae have evolved the condition, suggesting a possible close relationship betweenOnocleaceae andBlechnaceae.[1]
Its importance has been disputed -Copeland for example, considered it taxonomically important,[2] whereasTryon and Tryon[3] and Kramer[4] all stated that the importance can only be judged in relation to other characteristics.[1]