Many ferns are popular houseplants, and if you have seen one for yourself, you might have noticed how some leaves have brown patches underneath. Those brown patches are actually spores. Spores allow ferns to reproduce and form new individuals.
Ferns are pteridophytes or spore-bearing Vascular Plants. Their spores predate seeds and pollen, reproductive structures that emerge later on in evolutionary history. Yes, that means ferns have existed long before Plants with flowers, fruits, or cones.
In the following, we will discuss what pteridophytes are, describe their morphology and life cycle, and go through the different types of pteridophytes.
What is the definition of pteridophytes?
Ferns and fern allies are collectively known as pteridophytes. The below shows the definition of pteridophytes.
What are examples of common pteridophytes?
Pteridophytes grow as herbs, trees, epiphytes, and floating plants, inhabiting both terrestrial and freshwater environments. Many examples of common pteridophytes species, including sword ferns, stag horn ferns, and bird's nest ferns are popular house plants.
There are around 12,000 extant species of pteridophytes. They are the second most diverse group of vascular plants, outnumbered only by Angiosperms (flowering plants).
We will discuss more examples later when we go through the different types of pteridophytes.
What are the characteristics of pteridophytes?
What distinguishes pteridophytes from other plants? What are the characteristics of pteridophytes? First, we will discuss the morphological features of pteridophytes. Then we will go through its life cycle and Reproduction.
The morphological features of pteridophytes
Pteridophytes, along with seed plants, are vascular plants. The presence of vascular tissues distinguishes pteridophytes from Bryophytes, like mosses and liverworts, which lack such tissues.
However, they are similar to Bryophytes in that they sexually reproduce via spores, which means they typically require moisture for fertilization (as we will see in the pteridophyte life cycle later).
Such spore-bearing characteristics also distinguish them from seed plants–Angiosperms (flowering plants) and Gymnosperms (nonflowering plants)--which are also vascular plants but reproduce via seeds.
Whereas a seed is a multicellular unit containing a plant embryo and its nutrients, a spore is a microscopic single-celled reproductive unit.
Most pteridophytes have thin, wiry roots. Many ferns arise from a short underground stem called a rhizome, while others develop from a short vertical stem at or near the surface of the soil.
Some also have true stems or trunks, as in the case of tree ferns.
Pteridophytes usually have leaves called fronds. The main parts of the frond are the blade and the rachis.
Frond structures can be described as simple, pinnatifid, or pinnate.
Simple means that the blade is undivided.
Pinnatifid means that the blades are deeply lobed but not fully divided into pinnae.
Pinnate means that the pinnae are arranged on each side of the rachis.
The word ‘pteridophyte’ is derived from two Greek words: ptery-, which means winged, and phyto-, which means plant. This name describes the wing-like appearance of the fronds characteristic of most ferns.
In most ferns, leaves grow from apical meristems at the tip of each frond.
These Cells are protected by hairs, scales, and the curled-over spiral structure of the unrolling leaf tip, which gives rise to the distinctive spiral curl form at the tip of fern fronds. Once the formation of the blade is completed, the embryonic tip and spiral structure disappear.
If you take a closer look at the underside of a fern frond, you might notice brown patches. These are spore clusters called sori (singular: sorus). Each sorus is a collection of tiny spore sacs called sporangia which contain spores.
In many species, each sorus is enclosed in a thin membrane called indusium (plural: indusia). In other species, the sori are not covered by true indusia and are instead found beneath the curled-over frond margins, referred to as false indusia. Some species do not have indusia altogether.
In some fern allies, including horsetails, sporangia are enclosed in cone-like strobili (singular: strobilus) at the tip of stems.
A strobilus is a tightly-packed unit of sporangia-bearing leaves.
In some species, the sporangia are produced on only one portion of the frond, while in others, the sporangia-producing frond may appear different from other fronds. Fronds that produce sporangia are described as fertile, while those that do not are said to be sterile.
The shape of the sori varies from species to species. Along with the shape of the fronds, the shape and location of sori and indusia are important in identifying pteridophytes.
The life cycle and reproduction of pteridophytes
The pteridophyte life cycle and Reproduction is characterized by an alternation of generations, where haploid gametophytes and diploid sporophytes alternately produce each other.
Recall that haploid means having one set of Chromosomes, while diploid means having two sets (one from each parent).
The diploid sporophyte phase is dominant in the pteridophyte life cycle. Upon maturity, the diploid sporophyte undergoes Meiosis, producing haploid spores in the sori under the fronds. Haploid spores are so light that they can travel long distances carried by the wind.
Under the right conditions, these spores germinate and produce small haploid plants called prothalli (singular: prothallus).
Prothalli are typically small and heart-shaped, with a number of root-like hairs called rhizoids growing underneath.
Some prothalli contain chlorophyll and can conduct Photosynthesis, while others lack chlorophyll and instead take nourishment by absorbing decomposing organic matter.
Once established, prothalli undergo Mitosis to produce the haploid male and female gametophytes, or the sex organs that produce gametes (eggs and sperm):
The archegonium is the female gametophyte. It produces the ovule, which contains the egg.
The antheridium is the male gametophyte. It produces sperm.
The sperm is flagellated, allowing it to swim to the archegonium, where it will fertilize the egg. The sperm must swim on a layer of moisture to reach the egg. The fusion of the sperm with the egg forms the diploid zygote (fertilized egg), which undergoes Mitosis to produce the diploid sporophyte.
Homospory vs heterospory
Most pteridophytes are homosporous, producing spores of equal size that develop into gametophytes that produce both male and female gametes. The life cycle we have just discussed pertains to homosporous pteridophytes.
However, a few groups have two types of spores and are said to be heterosporous. Heterosporous plants have two kinds of spores: megaspores and microspores.
What are the different types of pteridophytes?
Pteridophytes are the oldest among land plants. Many of the different types of pteridophytes that emerged during the Devonian and Carboniferous periods (419.2 million to 298.9 million years ago) are now extinct.
Pteridophytes comprise two distinct evolutionary lineages: Lycopodiopsida (lycophytes) and Polypodiopsida (ferns).
Notice how we say “distinct”: while ferns and lycophytes are often grouped together into pteridophytes, researchers have found that ferns are more closely related to seed plants than to lycophytes.
Lycophytes
There are around 1,300 species of lycophytes, which can be subdivided into three orders: Lycopodiales (clubmosses), Isoetales (quillworts), and Selaginellales (spikemosses).
Clubmosses are typically homosporous and have underground gametophytes. On the other hand, spikemosses and quillworts are heterosporous and have reduced aboveground gametophytes.
Ferns
There are around 10,500 species of ferns, which can be subdivided into four subclasses:
Equisetidae (horsetails), Ophioglossidae, Marattiidae, and Polypodiidae.
There are around 30 extant species of Equisetidae, most of which live in moist habitats. Their leaves are typically reduced into scales or needles, which are usually non-photosynthetic. On the other hand, their stems are segmented and photosynthetic. They are typically homosporous.
Ophioglossidae consists of two subgroups of ferns and fern allies with very little complexity: Psilotales (whisk ferns), which lack true roots and leaves, and Ophioglossales (Adder’s Tongue), which are fern-like plants whose leaves consist of a sterile and fertile segment.
There are around 110 species under Marattiidae, an ancient group of ferns. Species under this subclass tend to have large vertical rhizomes with swollen leaf bases.
Examples of these include the Hawaiian pala (Marattia douglasii) and the Madagascar tree fern (Angiopteris evecta).
With over 10,300 species, most extant ferns fall under Polypodiidae. These are typically characterized by a horizontal rhizome from which fronds and roots grow. Fronds are usually coiled in the bud and uncurl as they mature. This leaf development is called circinate vernation.
Sword ferns (also called Boston ferns), bird’s nest ferns, and staghorn ferns are just some of the species under Polypodiidae that you may be familiar with.
Pteridophytes - Key takeaways
- Pteridophytes are spore-bearing vascular plants.
- Pteridophyte spores are found in clusters called sori underneath fronds.
- Pteridophytes are either homosporous or heterosporous.
- The diploid sporophyte phase is dominant in the pteridophyte life cycle.
- Pteridophytes comprise two distinct evolutionary lineages: Lycopodiopsida (lycophytes) and Polypodiopsida (ferns).
References
- Alexey Shipunov, “6.2: Pteridophyta - the Ferns,” Introduction to Botany, 3 Jan. 2019.
- Warren Wagner, Lower Vascular Plant | Definition, Taxonomy, Physical Characteristics, and Facts, Encyclopedia Britannica, 28 Sept. 2021.
- Ezcurra Ecological Research, Chapter 14. Pteridophytes: Ferns and Allies, May 2020.
- Jack Holt, PTERIDOPHYTA, Systematic Biology BIOL: 202, 17 Apr. 2013.
- PPG I, A Community-derived Classification for Extant Lycophytes and Ferns, Special Issue: Systematics and Evolution of Lycophytes and Ferns, Nov. 2016.
- Daniela Dutra Elliott and Paula Mejia Velasquez, Ferns, Botany in Hawaiʻi, 3 June 2021.
- Gary Fewless, Pteridophytes of Wisconsin: Basic Terms, 2022.
- US Forest Service, Fern Structure, 2022.
- University of California Museum of Paleontology, Introduction to the Psilotales: The Whisk Ferns, 2022.
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