Frond

frond [ frond ] noun: the leaf of a fern 

Although people may refer to the leaves of palms and palm-like woody plants (cycads) as fronds, some botanists restrict the term to ferns. Each crosier (fiddlehead) that emerges from a fern’s underground rhizome, is a coiled leaf or frond. The leafstalk that connects the frond to the rhizome is called a stipe, (in other plants it would be called a petiole). The upper leafy part of the frond is a lamina (blade), which includes the rachis (midrib) and the pinnae (leaflets).

Frond Type According to Leaf Dissection

Fronds can be undivided (simple or entire), lobed (pinnatifid), or divided into separate segments: pinnate (once divided), bipinnate (twice divided), tripinnate (thrice divided). When some or all of the divisions do not cut to the rachis then pinnatifid is added to the description: pinnate-pinnatifid, bipinnate-pinnatifid, tripinnate-pinnatifid.

Simple fronds of Asplenium scolopendrium americanum
(American hart’s tongue fern).
Photo courtesy of Linda Swartz

The pinnatifid frond (bases can be more pinnate)
of Onoclea sensibilis (sensitive or bead fern) is
deeply lobed almost to the rachis with opposite pinnae.
Photo © Mary Free

The pinnatifid frond of Woodwardia areolata (netted chain fern) is
deeply lobed almost to the rachis with alternate pinnae.
Photo © Elaine Mills

The pinnate frond of Asplenium platyneuron (ebony spleenwort) is
divided into segments completely separated from each other arranged alternately.
Photo © Mary Free

The pinnate fronds of Polystichum acrostichoides (Christmas Fern) are
divided into segments completely separated from each other arranged alternately.
Photo © Mary Free

Pinnate-pinnatifid frond dissection of
Matteuccia struthiopteris (ostrich fern).
Photo © Mary Free

Bipinnate frond dissection of Osmunda spectabilis (royal fern).
Photo © Mary Free

Bipinnate-pinnatifid frond dissection of Athyrium asplenioides (southern lady fern).
Photo © Elaine Mills

Tripinnate-pinnatifid fronds of Pteridium aquilinum (bracken fern). Blade divides into
3 triangular sections with basal pinnules distinct and upper portion of pinnae pinnatifid.
Photo © Elaine Mills

Frond Type According to Spore Production

Fronds are further distinguished based on how they display their fertile (reproductive) and sterile (vegetative) parts. In monomorphic ferns, like deciduous Adiantum pedatum (northern maidenhair fern) and Athyrium asplenioides (southern lady fern) and evergreen Dryopteris marginalis (marginal wood fern), fronds are virtually the same in appearance whether or not their undersides bear sori–groups of sporangia (spore-producing receptacles).

Fronds of monomorphic Adiantum pedantum (northern maidenhair fern) in May.
Photo © Mary Free

Underside of fertile pinna of
Adiantum pedantum (northern maidenhair fern) in September.
Photo © Mary Free

Fronds of monomorphic Athyrium asplenioides (southern lady fern) in April.
Photo © Elaine Mills

Underside of fertile pinnae of Athyrium asplenioides (southern lady fern) in October.
Photo © Elaine Mills

Fronds of monomorphic Dryopteris marginalis (marginal woodfern) in June.
Photo © Elaine Millls

Underside of fertile pinnae of Dryopteris marginalis
(marginal wood fern) in June.
Photo © Elaine Millls

In hemidimorphic ferns, a single frond may be divided into distinct portions that are either fertile or sterile. This kind of frond with two forms is called heteromorphic. On Polystichum acrostichoides (Christmas fern) fronds, the fertile upper pinnae are smaller and narrower. The sterile leaflets on Osmunda claytoniana (interrupted fern) fronds are interrupted mid-blade with green spore-bearing pinnae that ripen to brown, release their spores, and then fall off in summer. In Osmunda spectabilis (royal fern), the bead-like capsules containing spores appear in tassel-like clusters above the sterile egg-shaped leaflets on the lower two-thirds of the frond.

Sterile pinnae of Polystichum acrostichoides (Christmas fern)
with their unique sock or earlobe shape in April.
Photo © Mary Free

Fertile pinnae of Polystichum acrostichoides (Christmas fern) appear
at the top one-third of the frond, seen here from above and below in June.
Photo © Mary Free

Maturing fertile pinnae in between sterile pinnae of
Osmunda claytoniana (interrupted fern) in late April.
Photo © Mary Free

Bead-like, sori-containing capsules sit atop the sterile, egg-shaped leaflets
of Osmunda spectabilis (royal fern) in May.
Photo © Mary Free

In holodimorphic ferns, entire fronds are devoted solely to reproduction and those fertile fronds differ in appearance from the vegetative sterile fronds. Erect fertile fronds of Matteuccia struthiopteris (ostrich fern) arise in summer like beaded plumes amidst arching feathery sterile fronds, persist through winter, and release their spores the following spring. The sterile fronds of Onoclea sensibilis (sensitive fern) wither with the frost, but narrow panicles of fertile fronds remain erect through winter and release their spores in spring. Osmundastrum cinnamomeum (cinnamon fern) fertile fronds appear first in spring maturing from green to cinnamon and promptly wither after releasing their spores early summer. The sterile fronds they leave behind give a vibrant fall display. [In the plant examples below, the epithets of the species translate to their common names.]

Fertile fronds stand tall amidst the arching sterile fronds
of Matteuccia struthiopteris (ostrich fern) in September.
Photo © Elaine Millls

Fertile fronds of Matteuccia struthiopteris (ostrich fern)
that have persisted through winter in March.
Photo © Elaine Millls

Maturing fertile fronds stand behind the sterile frond
of Onoclea sensibilis (sensitive fern) in September.
Photo © Elaine Millls

Fertile fronds of Onoclea sensibilis (sensitive or bead fern)
that have persisted through winter in March.
Photo © Elaine Millls

Pinnate-pinnatifid sterile fronds encircle the cinnamon fertile fronds
of Osmundastrum cinnamomeum (cinnamon fern) in April.
Photo © Elaine Millls

Sterile fronds of Osmundastrum cinnamomeum (cinnamon fern)
put on a vibrant fall display in October.
Photo © Elaine L. Mills

References

Britton MR, Watkins JE, Jr. 2016. The economy of reproduction in dimorphic ferns. Annals of Botany. 118(6):1139-1149. doi: 10.1093/aob/mcw177.

Primitive (seedless) Vascular Plants. Ohio Plants.org  (https://ohioplants.org/ferns accessed March 2, 2021).

Watkins JE, Jr, Churchill AC, Holbrook NM. 2016. A site for sori: Ecophysiology of fertile–sterile leaf dimorphy in ferns. doi: 10.3732/ajb.1500505.

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