Pteridophytes (Spore Producing Plants: Ferns and More)

Pteridophytes are ancient plants commonly referred to as ferns or fern allies. In the context of evolution, pteridophytes fall between bryophytes and seed plants and share many characteristics with both of these groups. Ferns and allies can be found all over the world and are some of the most common plants in the understory of certain ecosystems. In addition, many ferns are grown ornamentally and are renowned for their beautiful foliage. 

What Are Pteridophytes? 

Pteridophytes are vascular plants that produce spores. These include ferns, horsetails, clubmoss, and spikemoss. Pteridophytes can be distinguished from other spore-producing plants – bryophytes – by their vascular tissue. In addition, they can be distinguished from other vascular plants – gymnosperms and angiosperms – by their lack of seeds. 

Pteridophytes Are Vascular Plants 

Pteridophytes are part of a large group of plants called tracheophytes. Tracheophytes are plants that have a vascular system, which allows for water and nutrients to be transported throughout the plant. Pteridophytes were the first group of plants to develop vascular tissue when they diverged from bryophytes millions of years ago. They contain two kinds of vascular tissue – xylem and phloem.

• Xylem is responsible for transporting water and minerals. 
• Phloem is responsible for transporting nutrients such as sugars and carbohydrates. 

Most Pteridophytes have bundled vascular tissue. That is, when looking at a cross-section of a stem, there are distinct bundles of xylem and phloem. Alternatively, some ferns possess vascular rings in the stem that surround the pith. Vascular bundles and vascular rings are akin to the vascular tissue organization of two angiosperm groups, monocots and dicots.

Tree fern stem showing vascular bundles in black.

Differences Between Pteridophytes and Seed Plants

Although ferns and fern allies have vascular tissue, they exhibit a vastly different form of reproduction as other vascular plants, gymnosperms and angiosperms. Pteridophytes don’t produce cones, flowers, or fruit, unlike these seed plants. Instead, Pteridophytes produce spores. Spores are living, single-celled structures that are essential to the life cycle of many plants.

Life Cycle of Pteridophytes

The life cycle of pteridophytes is very similar to that of other spore-producing plants. The next sections will examine how pteridophytes reproduce and the unique structures involved in the process. 

Alternation of Generations

Pteridophytes, like many other plants, exhibit true alternation of generations. This means that a plant must complete two distinct generations in order to complete its life cycle. 

Sporophyte

The main generation is the sporophyte, which is the dominant phase. The sporophyte is the only generation that is conspicuous. Thus, all ferns that we see growing in the forest represent the sporophyte generation. Sporophytes are diploid (2n) and contain two pairs of each chromosome in a cell. When a sporophyte is mature, it produces sporangia on the underside of fertile leaves or stems. Such fertile fronds are called sporophylls. Sporangia are structures in which spores are formed through meiosis. Spores are released from a sporangium, usually in dry conditions when the outer casing cracks and opens up. The spores are then dispersed by wind or water to start the next generation, the gametophyte. 

A pteridophyte that produces one type of spore is called homosporous. Homosporous plants produce only one type of spore that contains both male and female parts. The resulting gametophyte is monoecious. Conversely, a pteridophyte that produces two types of spores is called heterosporous. Heterosporous plants produce two kinds of spores – megaspores and microspores – which are female and male, respectively. The gametophytes that result from a heterosporous plant are thus dioecious, either female or male. 

Gametophyte

The gametophyte, sometimes referred to as a prothallus, is multicellular, microscopic, and generally short-lived. Unlike the sporophyte, a gametophyte reproduces sexually. It’s usually incapable of self-fertilizing, so it must reproduce with a nearby, genetically distinct individual. 

Gametophytes produce gametes (eggs or sperm) through mitosis. These gametes are haploid (1n) and contain only half of the genetic information of the plant. They are formed in structures called archegonia or antheridia. The archegonium forms the eggs, while the antheridium produces sperm. In the presence of water, a sperm uses its flagella to swim towards an archegonium, attracted by chemical signals. Once it arrives, it fertilizes the egg contained within. Sexual reproduction only occurs when water is present. For this reason, it’s rare to find pteridophytes in extremely dry places like deserts. 

The fusion of a sperm and an egg results in a zygote. This new structure is diploid, and through mitosis, it develops into a new sporophyte. Finally, the life cycle is complete.

Pteridophyte Groups and Diversity

There are approximately 12,000 species of pteridophytes. While not nearly as diverse as other groups of plants, like flowering plants, ferns and allies still exhibit a stunning array of forms and functions. 

Pteridophyte taxonomy has been in constant flux for quite some time. The group “Pteridophyta” was once used to encompass ferns and fern allies, but this group is no longer valid. The classification of Pteridophytes has changed a lot in recent years thanks to genetic and molecular analyses. Here, we follow the most recent and most widely accepted classification of pteridophytes made by the Pteridophyte Phylogeny Group. The following list of groups is not exhaustive, as there are several smaller orders and families of Pteridophytes not discussed.

Lycopodiopsida

The lycopods are the “allies” of ferns and fern allies and are sometimes referred to as lycophytes. Lycopods were the first group of pteridophytes to evolve, making them the oldest vascular plants. 

Lycopodiales (Clubmoss) 

Clubmosses (sometimes written as two words, club moss) make up the most ancient group of lycopods and originated around 380 million years ago. The order Lycopodiales contains just one family, Lycopodiaceae, which has around 400 species. Lycopodium is the largest genus in the family. 

In general, clubmosses are slender, herbaceous plants that have leaves called microphylls. Microphylls are usually compressed to the stems, creating scale-like structures and give the plant its prehistoric look. All clubmosses are homosporous, and spores are produced on a specialized structure called a strobilus. The strobilus is located on the ends of stems and gives clubmoss its name. 

Selaginella (Spikemoss)

Spikemosses are the other main group of lycopods. The order Sellaginellales contains just one genus, Selaginella, in which all spikemosses are contained. Spikemoss differs from clubmoss in having ligules, which are small structures between the leaf and stem. In addition, spikemosses are heterosporous. The general growth form of spikemoss is also different from clubmoss. They often creep along the ground and rarely exceed one meter in height. 

Polypodiopsida

Polypodiopsida, or ferns, is the main group of pteridophytes. There are several orders of ferns which contain a total of around 10,500 species. Ferns are generally distinguished by producing more complex leaves than lycopods. Fern leaves are called fronds and often branch multiple times. 

Equisetales

The order Equisitales contains plants people normally wouldn’t associate with ferns. However, it is within the group Polypodiopsida making it a fern in a taxonomical sense. The main genus, Equisetum, contains horsetails and scouring rushes. These plants are characterized by having a segmented stem with whorls of thin leaves at each node. They are often aquatic or weedy at many disturbed sites. All horsetails are homosporous and produce spores on a strobilus similar to lycopods. 

Psilotales

This group contains one family, Psilotaceae, and two main genera, Psilotum and Tmesipteris. These ferns are characterized by having very reduced leaves. Their sporangia are characteristic yellow balls located directly on the stems. 

Psilotum is commonly known as whisk ferns. These spindly ferns grow almost exclusively in tropical regions in the Americas and Asia. Tmesipteris, on the other hand, grows mainly in Australia, New Zealand, and New Caledonia.

Ophioglossales

Ophioglossales contains one family, Ophioglossaceae, otherwise known as tongue ferns. Tongue ferns differ from other types of ferns in two very interesting ways. First, many species of tongue ferns produce only one frond per year. Second, the gametophyte is often subterranean and gets all of its energy from mycorrhizal fungi.

Hymenophyllales

Hymenophyllales also contains only one family, Hymenophyllaceae. These ferns are commonly referred to as filmy ferns, due to their often semi-translucent, delicate fronds. Because the fronds are so delicate and prone to drying out, filmy ferns are usually restricted to habitats with abundant rain or running water.  

Salviniales

This group of ferns, called the water ferns, is unique in that all species are aquatic. They are also unique because they are all heterosporous, unlike most other ferns. One fern in this group, the mosquito fern (genus Azolla), is amongst the fastest-growing plants in the world. Another interesting member, Marsilea, resembles a clover and has been eaten by Aboriginal Australians for millennia.

Cyatheales

Cyatheales is one of the most unique groups of ferns, known as tree ferns. These massive plants are amongst the largest of the Pteridophytes, growing upwards of 10 meters (30 feet) tall. One of the most interesting things about this group is the fact that the tree-like habit is likely a case of convergent evolution. This tree-like growth is not exclusive to one group within Cyatheales, and many groups also contain more typical looking ferns.

Polypodiales

Polypodiales is by far the most diverse group of ferns. This Order alone contains 80% of existing fern species in 26 families. Ferns in this group all have an annulus, a specialized structure on sporangia that aids in spore dispersal. As the annulus dries, it eventually snaps off the sporangium, mechanically launching spores away from the plant. The annulus is unique to Polypodiales and distinguishes this group from all other ferns. A few of the most important families in this group are:

Dennstaedtiaceae

This family contains bracken fern (Pteridium aquilinum), a fern with a cosmopolitan distribution. Bracken fern’s ultra-light spores helped distribute the species across the globe.  

Pteridaceae

Pteridaceae contains maidenhair ferns (Adiantum), shoestring ferns (Vittaria), and brakes (Pteris). 

Dryopteridaceae

One of the largest fern families, Dryopteridaceae contains many diverse genera including Elaphoglossum, Polystichum, and Dryopteris. 

Conclusions

Despite their ancient history, ferns have persisted for millions of years and have evolved a wealth of growth forms. They represent a sort of transition between bryophytes and seed plants, making them the only seedless vascular plants in the world. They make up a vital part of many habitats and ecosystems across the globe. Although no Pteridophytes are cultivated as crop plants, there are many traditional ways to use them. In addition, the fragile beauty and prehistoric look of many Pteridophytes have made them centerpieces of various forms of art.