6.2.2: Ferns

Gametophyte Morphology

Fern gametophytes are reduced, thalloid, and heart-shaped. They are often referred to as a prothallus or prothallium.

Figure \(\PageIndex{1}\): Three fern gametophytes that have been stained and so appear blue. The images are labeled as follows: A) Prothallus, B) Archegonium, C) Rhizoid, D) Antheridium. Scale=0.525mm. The rhizoids and archegonia emerge from the underside off the thallus. Descriptive text: A points to the flat, heart-shaped thallus, B points to structures with elongated necks close to the rhizoids, C points to long thin projections, D points to small dark structures spread across the thallus. Scale bar = 0.525mm. The rhizoids and archegonia emerge from the underside of the prothallus. Photo by Jon Houseman, CC BY-NC, via Wikimedia Commons.

Figure \(\PageIndex{2}\): A section through a fern prothallus. The archegonia and rhizoids are located on the underside of the thallus. Each archegonium has a long neck. At the base of each archegonium, a single egg is housed. Photo by Maria Morrow, CC BY-NC.

Figure \(\PageIndex{3}\): The underside of a fern prothallus viewed under a microscope. The long necks of the archegonia are visible and appear dark, protruding outward. Long, thin rhizoids emerge near them. Photo by Maria Morrow, CC BY-NC.

Figure \(\PageIndex{4}\): A magnified view of a fern gametophyte that has both antheridia and archegonia. The antheridium (A) has many sperm inside (B) and does not appear three dimensional. The archegonium (C) is surrounded by enlarged cells--we are looking straight down the neck. At the center of the archegonium, there is only a single egg (D). Photo by Maria Morrow, CC BY-NC.

Figure \(\PageIndex{5}\): Fern gametophytes may develop either antheridia or archegonia based on environmental characteristics such as nutrients, competition, light, and antheridiogens. More nutrients, more light, less competition, and the absence of antheridiogen all select for the production of archegonia. Less nutrients, less light, more competition, and antheridiogen all select for the production of antheridia. Diagram by Jon Houseman, CC BY-NC, via Wikimedia Commons.

Sporophyte Morphology

Fern sporophytes are composed of megaphylls, often pinnately compound fronds that emerging as fiddleheads in the spring. Sporangia are produced in clusters called sori (sorus, singular) on the fronds.

Figure \(\PageIndex{6}\): Anatomy of a fern. The fern frond (D) is emerging from an underground, horizontal stem called a rhizome (C). The frond is twice pinnately compound. The leaflets (B, also called pinnae) branch off of a central stem called a rachis (A). Drawing by Pearson Scott Foresman, Public domain, via Wikimedia Commons with labels added by Maria Morrow.

Circinate Vernation

Circinate vernation is a term used to describe the development of the fern fiddlehead into a frond. Because plants grow apically, it is important to protect the apical meristems in growing organs (as we have seen in both axillary and terminal buds with the evolution of bud scales). The fiddlehead is essentially a structure that tucks away the growing tips of the fronds. As the frond develops, it gradually unfurls, releasing the tips last.

Figure \(\PageIndex{7}\): The two images above show fiddleheads of two different types of ferns, a deer fern on the left and a sword fern on the right. Each has the same basic shape--a long stalk that transitions in a tightly coiled knot containing the developing leaf tissues. Photos by Maria Morrow, CC BY-NC.

Figure \(\PageIndex{8}\): The two images above show the same species of fern, a five finger maidenhair fern. In the first image, the leaflets are partially curled. They are midway through the process of uncurling, called circinate vernation. In the second image, you can see the mature form of that fern frond, after circinate vernation is complete. Photos by Maria Morrow, CC BY-NC.

Sori

A sorus (plural, sori) is a cluster of sporangia, often protected by an umbrella-like structure called the indusium as the spores mature. Each sporangium is lined by an inflated strip of cells called an annulus. When the spores have matured, the cells in the annulus begin to dry out, causing the cells to collapse and pull the sporangium open, releasing the spores.

Figure \(\PageIndex{9}\): The underside of Polypodium frond. Each fuzzy-look orange region is a developing sorus (a cluster of sporangia). These sori are not protected by an indusium. Photo by Maria Morrow, CC BY-NC.

Figure \(\PageIndex{10}\): Cross section of a sorus. This cluster of sporangia is protected by a central umbrella-like structure called an indusium. Each sporangium is surrounded by a strip of inflated of cells called the annulus. When the cells in the annulus dry, it causes the sporangium to split open, releasing the spores. A=Epidermis, B=Vascular bundle, C=Annulus, D=Sporangium, E=Spore, F=Indusium. Photo by Maria Morrow, CC BY-NC.

Figure \(\PageIndex{11}\): The epidermis of the leaf (sporophyll) is labeled A. A large vascular bundle (B) is located in the center, as well as several others located along the length of the leaf. Under the leaf, a cluster of sporangia (D) are forming a sorus. Each sporangium is surrounded by a thickened layer of cells, the annulus (C). Within the sporangia, spores (E) are produced. The entire sorus was once covered by the residual structures labeled F, the indusium. Photo by Jon Houseman, CC BY-NC, via Wikimedia Commons.

Figure \(\PageIndex{12}\): Cross section of a Dicksonia sorus. In this fern, the cluster of sporangia is enveloped by an extension of the leaf (E), a false indusium. A=Epidermis, B=Annulus, C=Spore, D=Sporangium, E=False indusium. Scale=0.1mm. Photo by Jon Houseman, CC BY-NC, via Wikimedia Commons.

Full Life Cycle Diagram

Figure \(\PageIndex{13}\): The diagram above shows the life cycle of a Polypodium fern. The gametophyte generation (n) is shown in the top half of the diagram and the sporophyte generation (2n) in the bottom half. Starting from the release of haploid homospores, these spores grow by mitosis into bisexual gametophytes. The gametophyte is heart-shaped, thalloid, and produces root-like structures called rhizoids. The antheridium produces many swimming sperm that are dispersed by water into an archegonium. A sperm swims down the neck/venter of an archegonium and fertilizes the single egg produced there. This produces a diploid zygote, which is retained on the gametophyte. The zygote grows by mitosis within the archegonium, producing a sporophyll (frond). When fully developed, the sporophyte will likely have multiple fronds and a rhizome. Fronds start as fiddleheads and uncoil by circinate vernation. The frond on the left is producing sori on the underside of the leaflets. Each sorus is a cluster of sporangia, which is protected by an indusium. Meiosis happens within the sporangia. Each sporangium has an inflated annulus to help release the spores when conditions are right. One of these has torn open to release its haploid homospores, which brings us back to the beginning. Diagram by Nikki Harris CC BY-NC with labels added.

Note: Images by Jon Houseman are licensed in Wikimedia as CC BY-SA. In an email with Maria Morrow, he agreed that we could use the CC BY-NC license for these images.