Since ferns, unlike flowering plants, bear spores on the leaf 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 the genus Dryopteris, other than the mere presence of the sori, or fruit-dots, on the back of the fronds.
Others, such as some species of Blechnum and Woodwardia, have fertile fronds that are markedly taller than the sterile.
Still others, such as Osmunda cinnamomea (Cinnamon fern), or plants of the family Onocleaceae, have fertile fronds that are completely different from the sterile.
Only members of the Onocleaceae and Blechnaceae exhibit a propensity towards dimorphy, while no member of the Athyriaceae is strongly dimorphic, and only some representatives of the Thelypteridaceae have evolved the condition, suggesting a possible close relationship between Onocleaceae and Blechnaceae.