Plant Kingdom · Lesson 6 of 9
Pteridophytes: Vascular Plants and Heterospory
“Explore the vascular sporophyte, prothallus life cycle, sporangia, homospory, heterospory and seed-habit precursors.”
• Identify why pteridophytes are vascular plants and distinguish their dominant sporophyte from bryophytes. • Explain sporangia, sporophylls, strobili, prothallus and water-dependent fertilisation. • Trace pteridophyte alternation of generations with ploidy at every stage. • Compare homospory and heterospory and explain the evolutionary significance of retained female gametophytes. • Recognise representative pteridophyte groups including Selaginella, Equisetum and ferns.
What changed when plants developed vascular tissues?
A fern in a damp forest has roots anchored in soil, a stem often growing as a rhizome and leaves that may form large fronds. Its main green body differs from the small leafy gametophyte of a moss because the fern is predominantly a diploid sporophyte. Pteridophytes, including ferns and horsetails, have true vascular tissues and differentiated organs. Those tissues help conduct water, minerals and organic products through the plant body.
A vascular tissue specialised mainly for transporting water and mineral nutrients and often providing mechanical support.
A vascular tissue transporting photosynthetic products and other organic substances between sources and sinks.
The presence of xylem and phloem allows greater differentiation of root, stem and leaf functions. Roots take up water and help anchor the plant, leaves capture light and exchange gases, and stems connect these regions. Vascular support permits larger body size in many species, although pteridophytes range from modest ground plants to impressive tree ferns. Vascularity does not imply seed formation: these plants still use spores.
The pteridophyte sporophyte is the dominant plant
The visible fern frond belongs to a 2n sporophyte. Spores form on this body within sporangia, often arranged on specialised leaves or leaf-like structures called sporophylls. Certain species cluster sporophylls into strobili or cones. This is a spore-bearing cone, not the seed-bearing woody cone of a gymnosperm: the same general shape can support quite different reproductive processes.
A structure in which spore mother cells undergo meiosis to produce haploid spores.
A modified leaf or leaf-like structure associated with sporangia.
A compact arrangement of specialised reproductive leaves or appendages along an axis, commonly called a cone.
Ferns may show clusters of sporangia on the underside of fronds, whereas Selaginella and Equisetum often display strobili. The chapter also contrasts small microphylls as found in Selaginella with large fern macrophylls. Leaf size and vascular arrangement help distinguish lineages, but a particular plant must be examined carefully rather than classified solely by the word 'small'.
From a spore to a prothallus
A generally small, free-living, photosynthetic haploid gametophyte produced by germination of a pteridophyte spore.
A diploid spore mother cell within the sporangium undergoes meiosis, forming haploid spores. Under suitable conditions, a spore germinates into a small multicellular gametophyte known as a prothallus. In many ferns, the prothallus is green, delicate and capable of photosynthesis. It is much less conspicuous than the leafy diploid sporophyte and typically favours moist, protected locations.
The prothallus may form antheridia and archegonia. Antheridia produce motile male gametes; archegonia contain eggs. A film of water is needed for male gametes to reach the archegonium. The zygote begins developing into a new diploid sporophyte, which establishes its own conducting tissues and eventually becomes the dominant, free-living plant.
This cycle explains why vascular pteridophytes are not as independent of external water for fertilisation as seed plants. The mature sporophyte transports water through xylem, but the gametophyte stage often remains small and needs humid environments. Body transport and sexual fertilisation are separate biological challenges; solving the first does not automatically solve the second.
Life-cycle diagram and chromosome changes
Homospory: producing one main kind of spore
Producing one kind of spore, without a distinct large megaspore and small microspore pair.
Many ferns and other pteridophytes are homosporous. Spores formed in their sporangia are broadly similar in kind and often give rise to gametophytes capable of producing both male and female reproductive structures, though reproductive arrangements vary. Homospory does not mean a plant cannot reproduce sexually; the life cycle still contains gametes, fertilisation and a zygote.
A common error is to equate a spore with a gamete. Both can be haploid, but they have different roles. A spore germinates and divides to form a gametophyte; a gamete fuses with another gamete. Homospory refers to spore categories, not to whether two gametes look alike.
Heterospory: microspores and megaspores
Producing two distinguishable kinds of spores: microspores and megaspores, which give rise to male and female gametophytes.
Selaginella and Salvinia are important heterosporous examples in the source. Smaller microspores develop into male gametophytes, whereas larger megaspores develop into female gametophytes. This division of reproductive tasks permits differentiation in the amount of stored resources and protective association with the parent plant.
In some heterosporous pteridophytes, female gametophytes remain associated with the parent sporophyte for varying periods. The fertilised egg then develops into a young embryo within that retained gametophyte. The retention and nourishment of reproductive structures are evolutionary features relevant to understanding the later seed habit. However, Selaginella and Salvinia do not produce true seeds in the sense characteristic of gymnosperms and angiosperms.
| Feature | Homosporous pteridophyte | Heterosporous pteridophyte |
|---|---|---|
| Spore kinds | One main spore category | Microspores and megaspores |
| Gametophyte outcome | Often potentially bisexual prothallus | Distinct male and female gametophytes |
| Representative examples | Many common ferns | Selaginella, Salvinia |
| Evolutionary significance | Independent gametophyte often free-living | Specialisation, retention and protection of female generation can develop |
| Seed production | No seeds | No true seeds, although some features precede seed habit |
Representatives and traditional classes
The chapter groups pteridophytes into four traditional classes: Psilopsida, Lycopsida, Sphenopsida and Pteropsida. Examples given are Psilotum, Selaginella or Lycopodium, Equisetum, and ferns such as Dryopteris, Pteris and Adiantum. These names are practical historical labels. Modern evolutionary classifications revise some relationships, but students should recognise the textbook grouping and typical body forms.
| Traditional class | Representative | Useful observation |
|---|---|---|
| Psilopsida | Psilotum | Simplified branching shoot organisation |
| Lycopsida | Selaginella, Lycopodium | Microphylls; Selaginella has heterospory |
| Sphenopsida | Equisetum | Jointed shoots, nodes and internodes, strobilus |
| Pteropsida | Dryopteris, Pteris, Adiantum | Large fern fronds and sporangia |
The source illustration also includes Salvinia, a floating aquatic heterosporous fern. Equisetum displays an upright segmented stem with nodes and internodes and a terminal strobilus. Selaginella is often identified by its small leaves and branching body. These figure labels serve more reliably than memorising the outline of an unlabelled silhouette.
Importance and environmental limits
Ferns are grown ornamentally, and some pteridophytes are used in traditional medicines or for soil stabilisation. They contribute to shaded woodland ecosystems, and some aquatic forms influence freshwater habitats. Their dependence on damp environments for gametophyte development and fertilisation helps explain why many species occupy restricted habitats, though some pteridophytes tolerate relatively dry sites.
Problem
A specimen has true roots and xylem, sporangia on specialised leaves, and two spore sizes. Its female gametophyte develops from the larger spore. Classify it and explain whether it already has seeds.
- 1.True vascular tissues and sporangia with no seeds identify a pteridophyte.
- 2.Two spore categories indicate heterospory: megaspores make female gametophytes and microspores make male gametophytes.
- 3.The specimen may resemble Selaginella or Salvinia. Heterospory anticipates aspects of seed evolution, but does not itself produce true seeds.
Quick Quiz
Quiz
What distinguishes the conspicuous pteridophyte plant from the main bryophyte body?
What is a prothallus?
Where are spores formed in a pteridophyte?
Which two pteridophytes are commonly used as examples of heterospory?
Which development is significant as a precursor to seed habit?
What structure is the diploid dominant phase of a typical fern?
What is incorrect about calling a microspore a male gamete?
Practice Problems
- Describe the pteridophyte alternation of generations, identifying meiosis, fertilisation and ploidy.
- Explain why a fern is more structurally independent on land than a moss but still depends on water for sex.
- Compare homospory and heterospory using two examples of the latter.
- Why is heterospory important for understanding the evolution of seeds?
- Identify one representative of Psilopsida, Lycopsida, Sphenopsida and Pteropsida.
The dominant 2n sporophyte makes haploid spores by meiosis in sporangia. Spores form free-living n prothalli, which produce gametes. Water allows sperm to reach the egg; fertilisation restores 2n and the embryo develops into a sporophyte.
Key Takeaways
Pteridophytes include ferns and horsetails and have true vascular tissues plus differentiated roots, stems and leaves. The dominant phase is a diploid sporophyte (2n); meiosis in sporangia forms haploid spores. Spores produce small free-living gametophytes or prothalli (n), which require moisture for fertilisation. Homosporous plants form one main spore kind; heterosporous Selaginella and Salvinia form microspores and megaspores. Retention and protection of the female gametophyte in some heterosporous plants foreshadow seed habit but do not make true seeds.