Word of the Week: Frond

frond [ frond ] noun: the leaf of a fern

 

 

Diagram by Mary Free

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.

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).

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.

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.]

References

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

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

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

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