Facts About Ferns and the Western Sword Fern in British Columbia
Interesting Plants in the Wild and in Gardens
Ferns are attractive and interesting plants that grow very well in British Columbia, where I live. One common species here on the southwest coast of the province is the western sword fern, or Polystichum munitum. It's a native plant that grows in the wild and is also a valued addition to gardens and landscaped areas.
The dominant feature of most ferns is the frond. The frond consists of a stipe (the petiole or leaf stalk) that emerges from the ground and bears a large blade. The blade is divided into leaflets, or pinnae. The pinnae may be further divided, producing a beautiful frond. Some ferns have an atypical structure, however.
Ferns are vascular plants, which means they contain vessels that conduct nutrients and water. They don’t produce flowers or seeds, however. Instead, their reproduction involves spores. They usually live in moist and partially shaded habitats, though some live in drier areas. The western sword fern generally grows in the shade of trees and in the understory of forests.
Unfortunately, multiple classification systems for ferns exist. One system places the majority of ferns—including the western sword fern —in the phylum or division known as the Pteridophyta. Pteridology is the study of ferns.
Features of Ferns
The blade of the western sword fern is divided into pinnae. In some ferns (though not in the sword fern), each pinna is divided into pinnules. The pinnules are divided yet again in certain species. The result is an attractive frond with a lacy or feathery appearance.
The leaf part that is referred to as the midrib in flowering plants is called the rachis in ferns. The short stem at the base of the frond is the stipe. The stipes of a fern emerge separately from the ground. They come from the rhizome, which is a specialized underground stem that produces roots and shoots.
A young frond starts its life as a fiddlehead. This is a stalked structure with a curled head. The head slowly uncurls to reveal the growing frond. The structure is called a fiddlehead because it resembles the head of a violin, or fiddle. It's also known as a crozier after the curved or curled end of a bishop's staff.
The sori or reproductive structures of a fern can't be seen unless the underside of a frond is viewed. To get the photo above, I supported the end of a frond on another plant so that the sori were visible.
Production of Spores
There are two plant forms in a fern’s life cycle. The large plant that we call a fern is known as the sporophyte because it produces spores. The small prothallus or gametophyte stage of the plant that is produced from a spore is often overlooked. It's heart or kidney shaped and produces the male and female gametes (the sperm and the eggs).
If you look on the underside of the pinnae of many ferns, you'll see rows of small spherical or elongated structures. These structures are called sori. They contain sporangia, which are sacs containing spores. The sori are light green at first and turn brown and grainy when mature. In some ferns, each sorus is covered by a protective layer of tissue called the indusium, at least when the sorus is young. The sporangia burst open when they're mature, releasing the spores into the environment. The spores are then transported to new areas by air currents.
Some ferns have two types of fronds—sterile ones and fertile ones. An example of a fern with these fronds is the deer fern (Blechnum spicant), which grows in my part of the world. The fertile fronds bear sori while the sterile ones don't. A photo of the deer fern is shown below.
Release of Spores From Sporangia
Production of Gametes
When a fern spore lands in a suitable habitat, it produces a prothallus. Unless people are growing ferns from spores in their garden or home, the prothallus probably won't be noticed because it's so small. It's only 2 to 8 mm wide. It's green and performs photosynthesis. It has structures called rhizoids on its lower surface that absorb water and minerals.
The prothallus has archegonia (female organs) and antheridia (male organs) on its lower surface. The archegonia produce egg cells and the antheridia produce sperm. In some ferns, the archegonia and antheridia are produced in different prothalli, which must lie close together in order for fertilization to be successful. Several mechanisms in ferns enable cross-fertilization to occur between different prothalli.
When sperm are released from the antheridia, they swim to archegonia to reach the eggs. Each sperm has multiple flagella, which beat in order to move the sperm. The movement requires a film of water on the surface of the prothallus and on a neighbouring one if it's involved in fertilization. A sperm swims down the neck of an archegonium to reach an egg.
An egg and a sperm join to make a zygote, which produces a new sporophyte. Despite the presence of multiple sperm and eggs in a prothallus, it produces only one sporophyte. Once the sporophyte is sufficiently mature, the prothallus dies and disintegrates.
The prothallus of Onoclea sensibilis is shown at the bottom of the plant above. The top part shows a young sporophyte growing from the prothallus. The sporophyte is known as the sensitive fern because it's very sensitive to the first frost in the fall.
Haploid and Diploid Stages of the Life Cycle
The life cycle of a fern can be summarized as follows.
- The spores, prothallus, eggs, and sperm of ferns are haploid. The term "haploid" means that the structures have one set of chromosomes.
- When an egg and sperm join, they make a diploid zygote. Diploid cells have two sets of chromosomes. The zygote produces the sporophyte, or the familiar fern, whose cells are also diploid.
- The symbol n is used to represent haploid cells and 2n to represent the diploid ones.
- The mature diploid sporophyte produces haploid spores by a process called meiosis. This is the same process by which eggs and sperm are made in us.
Some Unusual Ferns
- In tree ferns, a trunk bears the fronds. The height of tree ferns varies considerably and depends on the species. Some grow much taller than a human being. Unlike the case in trees, the trunk of a tree fern is composed of a dense mass of rhizomes and modified roots.
- Ferns in the genus Azolla live in fresh water. The species in the genus are known as mosquito ferns or as duckweed ferns. They have some atypical features and look more like duckweed plants than ferns.
- The leaves of hart's tongue fern (Asplenium scolopendrium) have no pinnae. They look somewhat like the tongue of a hart, or an adult male deer, which gave the plant its name.
- The resurrection fern (Polypodium polypodioides) grows on trees without harming them and is therefore classified as an epiphyte. It gets its common name from the fact that it shrivels when it dries out and appears to be dead. When it absorbs water it expands and becomes active again.
- The Japanese climbing fern (Lygodium japonicum) resembles a vine. It both climbs and twines, forming dense mats over shrubs and trees. It's native to Asia and Australia but has been introduced to the United States. In some parts of the country, it's considered to be a noxious weed.
Ferns are one of the oldest groups of plants on Earth, with a fossil record dating back to the middle Devonian (383-393 million years ago).— Jerald Pinson, American Fern Society
The Western Sword Fern
The western sword fern is an evergreen plant belonging to the wood fern family, or the Dryopteridaceae. It's often a tall plant that may reach five feet in height in suitable climates and habitats. It forms clumps as it grows. The stipes emerge from a central area known as the crown. The crown consists of a dense and woody mass of rhizomes. According to the Royal BC Museum, in a mature plant the crown may reach a length of half a metre. Roots extend from the crown into the soil.
The plant's distribution extends beyond British Columbia. It's found in states along the Pacific Coast of North America from Alaska to California. According to the USDA (United States Department of Agriculture), it's also found in a few states further to the east.
In British Columbia, the plant is abundant in the understory of some forests. Though it grows best in shady and moist areas, it can tolerate some sun. Cultivated plants that are exposed to more sun than wild forms tend to develop shorter, more erect, and denser clumps.
The western sword fern is often referred to as simply "sword fern". This name is also used for Nephrolepis exaltata, a tropical fern that is popular as a house plant. One cultivar of the species is known as the Boston fern.
Identifying the Plant
The pinnae of the western sword fern are pointed and toothed. They are attached to the rachis in an alternate pattern. In the upper part of the frond, the pinnae become successively shorter towards the tip. Like the pinnae, the tip of the frond is pointed. The fronds in a clump often spread outwards from the central area. Their stipes are brown and hairy.
It may initially look as though the base of a pinna is completely attached to the rachis. A closer examination will reveal that it's actually joined via a very short stalk on one side of the base, as shown in the photo above. The other side of the pinna base is lobed. These features are most obvious in some of the upper pinnae in the photo.
Young western sword ferns are sometimes confused with deer fern. With a little practice and careful attention to details, though, it's easy to tell the species apart. The pinnae of deer ferns are not toothed. In addition, the entire base of each pinna is attached to the rachis and the characteristic lobe at the base of a sword fern pinna is missing.
The deer fern has sterile fronds and fertile ones. The fertile fronds are taller than the sterile ones and have narrow pinnae. The pinnae bear sori on their undersurface. The black and upright stalks in the photo above are part of the fertile fronds. The green lines extending from them are the pinnae.
More Facts About the Western Sword Fern
- The genus name Polystichum is derived from Greek and means "many lines". The lines referred to are the rows of sori on the undersurface of the frond. These are especially noticeable when the sori are mature and brown.
- The species name means "armed". The word refers to the teeth on the edges of the pinnae.
- The indigenous people of British Columbia once roasted western sword fern rhizomes and then peeled and ate them. This seems to have been done only in famine situations, however. It's currently recommended that people don't eat the rhizomes. They may not be safe to ingest.
- The fronds of the fern were once used to line baskets and baking pits.
- Today the fronds of cultivated plants are used in flower arrangements.
The Beauty of Ferns
Ferns are lovely to see in landscaped areas and can often make beautiful house plants. My favourite types are the wild ones, however. Cultivated sword fern clumps are sometimes tidier than wild ones, but I prefer to see the ones living a natural life in the wild.
Ferns can be impressive plants, especially when they're large and growing in a group. They don't have the colour variety of flowers or the size and features of some trees, but I think they have much to offer us. The attractive fronds and interesting features of many fern species are enjoyable to observe and explore.
- Fern structure from the United States Department of Agriculture (USDA) Forest Service
- Information about ferns from the American Fern Society
- Sword Fern facts from the Royal BC Museum
- Information about Polystichum munitum from Fire Effects Information System (a USDA website)
- The sword fern in coastal BC from Central Coast Biodiversity
Questions & Answers
How far will a fern's roots extend from the crown?
According to the U.S. Forest Service, if the soil has the right conditions (moist but well drained and at least a moderately fine texture) the fern can have a rooting depth of three feet or more.
Are there native varieties of Azolla in British Columbia?
Three species of Azolla are known to occur in British Columbia. All of them have a limited distribution.
Azolla filiculoides is known as the large or Pacific mosquito fern and is believed to have been introduced from the eastern United States. It’s found in a limited area in the south-central part of the province and is occasionally seen in the southwestern part.
The eastern or Carolina mosquito fern (Azolla caroliniana) is also found in BC and is believed to have been introduced from the eastern U.S. It’s found in the southwestern part of the province.
The third species found in the province is the Mexican mosquito fern or Azolla mexicana. This is a native species of British Columbia. The government has classified it as a red-listed taxon, which means that its population is threatened. It’s found in the southern central part of the province.
© 2018 Linda Crampton