Pteridophytes

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.

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).

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.

Pteridophytes usually have leaves called fronds. The main parts of the frond are the blade and the rachis.

• The blade is the leafy part of the frond. Many species have blades divided into leaflets called pinnae (singular: pinna).

• The rachis is a tough vascularized stalk to which the blade is attached.

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.

  • Each pinna can be further divided into smaller segments called pinnules. 2-pinnate (bipinnate) have fronds divided two times, while 3-pinnate (tripinnate) have fronds divided three times.

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.

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.

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).

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.

• Megaspores form female gametophytes. Megaspores also store nutrients for the developing embryo.

• Microspores form male gametophytes.

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).

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.

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.