Fern Structure

Ferns can have some very unusual forms and structures. The following describes fern structure and forms that people typically encounter.

Leaves

The leaves of ferns are often called fronds. Fronds are usually composed of a leafy blade and petiole (leaf stalk). Leaf shape, size, texture and degree of complexity vary considerably from species to species.

A fern leaf.
A fern leaf or frond.

Parts of a fern leaf.
Parts of a fern leaf.

The midrib is the main axis of the blade, and the tip of the frond is its apex.

The blade may be variously divided, into segments called pinnae; single leaflets are pinna. Pinna may be further divided, the smallest segments are pinnules.

Fiddleheads

As new fronds emerge, generally in the spring, they unroll, these unrolling fronds are called fiddleheads.

Alaska hollyfern (Polystichum setigerum).
Fiddleheads of Alaska hollyfern (Polystichum setigerum) just beginning to unroll.

Lady fern (Athyrium filix-femina).
Unrolling fiddleheads of a lady fern (Athyrium filix-femina).

Leaf Divisions

Depending on the species, fern leaves display a wide array of divisions. Various degrees of leaf divisions are shown in this series of frond silhouettes.

Fern leaf silhouettes: simple, pinnatified, pinnate, pinnate-pinnatified, 2-pinnate, and 2-pinnate-pinnatified.

Simple

The fronds are undivided.

Heart’s tongue fern (Asplenium scolopendrium var. americanum)
Heart’s tongue fern (Asplenium scolopendrium var. americanum). Photo by D.J. Evans, New York Natural Heritage Program.

Pinnatifid

The frond is divided into segments divided from each other almost to the rachis.

Sensitive fern (Onoclea sensibilis)
Sensitive fern (Onoclea sensibilis).

Licorice fern (Polypodium glycyrrhiza)
Licorice fern (Polypodium glycyrrhiza).

Pinnate

The frond is divided into segments completely separated from each other.

Maidenhair spleenwort (Asplenium trichomanes)
Maidenhair spleenwort (Asplenium trichomanes).

Further Divided

Many ferns are known for their lacy appearance, these ferns have fronds that are even further divided.

  • 2-pinnate (bipinnate): fronds are divided two times.
  • 3-pinnate (tripinnate): fronds are divided three times.
  • In cases were these secondary divisions do not cut to the rachis or the axis of the pinna the term pinnatifid is added to the degree of cutting to describe this type of frond dissection.

Examples of ferns displaying various degrees of leaf divisions:

Beech fern (Phegopteris connectilis)
Pinnate pinnatifid - Beech fern (Phegopteris connectilis).

Northern wood fern (Dryopteris expansa)
Bipinnate pinnatifid - Northern wood fern (Dryopteris expansa).

Dimorphic Fronds

Some ferns have two kinds of fronds: fertile fronds (leaves with sporangia) and sterile fronds (leaves lacking sporangia). Ferns with two kinds of leaves are referred to as dimorphic. Examples of dimorphic ferns are deer fern (Blechnum spicant) and cinnamon fern (Osmunda cinnamomea).

Deer fern (Blechnum spicant).
Deer fern (Blechnum spicant).

Cinnamon fern (Osmunda cinnamomea).
Cinnamon fern (Osmunda cinnamomea). Photo by Linda Swartz.

Other ferns, such as the moonworts have sterile pinnae and fertile pinnae on the same leaf. This can be seen in the moonwort fern (Botrychium lunaria).

Fern Sori

Sori (singular: sorus) are groups of sporangia (singular: sporangium), which contain spores. Sori are usually found on the underside of the blade. Young sori are commonly covered by flaps of protective tissue called indusia (singular: indusium). See the following graphic.

Three images of increasing magnification of the underside of a fern leaf showing sori, sorus, and sporangium.

Sori can vary considerably in shape, arrangement, location and covering depending on the kind of fern. These differences can be useful for identifying ferns. However, depending on the time of year, sori and indusia may not be useful characters because they may be too immature or too mature to be diagnostically useful.

The following are some of the more common kinds of sori.

Underside of a fern leaf, sporangia with no indusium.
Sori without indusia. The sori of polypody ferns do not have indusia. Here we see the sporangia with no indusium.

Underside of a fern leaf, sori with umbrella-shaped indusia.
Sori with umbrella-shaped indusia. The indusium is round, shaped like a tiny umbrella and attached to the leaf from the middle. These sword fern indusia do not quite reach the edge of the sori. Individual sporangia are easily visible around the edge of each sorus.

Underside of a fern leaf, sori with hood-like indusia.
Sori with hood-like indusia. The indusium is attached at the lower edge and partially under the sorus. The hoodlike indusia of fragile fern are easy to see early in the season.

Underside of a fern leaf, sori with hood-like indusia that are shredding.
Later in the season fragile fern’s indusia shread and become difficult to see.

Underside of a fern leaf, sori with kidney-shaped indusia.
Sori with kidney-shaped indusia. Northern wood-ferns have kidney-shaped indusia that are attached to the bottom of the frond by a narrow band of tissue.

Underside of a maidenhair spleenwort fern leaf, sori with linear indusia.
Linear sori with linear indusia. The tiny fronds of maidenhair spleenwort ferns bear few linear sori on their undersides. Note the tiny black spores resting on the frond. Asplenium trichomanes ssp. densum, Forest and Kim Starr, Starr Environmental, Bugwood.org.

Underside of a western maidenhair fern leaf.
The sori of western maidenhair ferns are covered by the folded-over end of the pinna.

Sori with false indusia. False indusia are not formed of specialized tissue (as are true indusia), but are leaf tissue rolled or folded over the sori. They can be marginal, along the side of the pinna, or at the tip of the pinna as in the maidenhair ferns.

Fern Stems and Roots

Fern stems (rhizomes) are often inconspicuous because they generally grow below the surface of the substrate in which the fern is growing. This substrate can be soil, moss or duff. People often confuse rhizomes with roots. Fern roots are generally thin and wiry in texture and grow along the stem. They absorb water and nutrients and help secure the fern to its substrate.

Stems can be short-creeping with fronds that are somewhat scattered along the stem, such as the fragile fern; or, stems can be long-creeping resulting in fronds scattered along the stem, exemplified by the licorice fern.

Licorice fern (Polypodium glycyrrhiza)
Licorice fern (Polypodium glycyrrhiza) has a long creeping rhizome (stem) with relatively widely scattered fronds (e.g. long-creeping). Note the wiry roots also growing from the rhizome.

Licorice fern (Polypodium glycyrrhiza)
The rhizome of this live licorice fern grows under a thin layer of moss and is tightly attached to an alder tree. Plants growing on other plants are called epiphytic plants.

Stems can be vertical, producing rosettes of leaves, as displayed by the sword ferns.

Kruckeberg’s hollyfern (Polystichum kruckebergii)
Pressed specimen of Kruckeberg’s hollyfern (Polystichum kruckebergii) showing the entire vertical rhizome (stem) and attached roots.

Braun’s hollyfern (Polystichum braunii)
Braun’s hollyfern (Polystichum braunii) showing the distinct rosette of fronds characterizing vertical stems.

Next: Fern Reproduction…