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UPSC MainsBOTANY-PAPER-I201915 Marks
Q8.

What are the critical features of heterospory and seed habit in pteridophytes.

How to Approach

This question requires a detailed understanding of evolutionary advancements in pteridophytes. The answer should begin by defining heterospory and the seed habit, then explain their occurrence, advantages, and evolutionary significance within pteridophytes. Focus on specific examples like *Selaginella* and *Isoetes* to illustrate the features. A comparative approach highlighting the differences from homospory will be beneficial. Structure the answer into introduction, detailed features of heterospory, features of seed habit, and conclusion.

Model Answer

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Introduction

Pteridophytes, representing a significant evolutionary leap from bryophytes, exhibit a diverse range of reproductive strategies. While most pteridophytes are homosporous, producing spores of a single type, a crucial evolutionary innovation observed in certain groups is heterospory. Heterospory, the production of spores of two distinct types – microspores and megaspores – is considered a pivotal step towards the evolution of the seed habit. This transition, observed in plants like *Selaginella* and *Isoetes*, marks a significant departure from the ancestral condition and provides insights into the origins of seed plants. The seed habit, involving the retention of megaspores within the megasporangium, further enhances protection and facilitates dispersal.

Heterospory in Pteridophytes

Heterospory is the production of two types of spores: microspores and megaspores. This phenomenon is observed in several genera of pteridophytes, including *Selaginella*, *Isoetes*, *Salvinia*, and *Azolla*.

  • Microspores: These are smaller spores produced in microsporangia. They develop into male gametophytes, which produce antherozoids.
  • Megaspores: These are larger spores produced in megasporangia. They develop into female gametophytes, which produce archegonia.

The advantages of heterospory include:

  • Reduced chance of self-fertilization: Different spore types promote cross-fertilization, increasing genetic diversity.
  • Enhanced protection of the female gametophyte: The megaspore wall provides better protection to the developing female gametophyte.
  • Increased efficiency of fertilization: Specialized male gametophytes deliver sperm directly to the egg.

Examples:

  • Selaginella: Exhibits a clear distinction between microsporangia and megasporangia, often clustered into strobili.
  • Isoetes: Possesses microsporangia and megasporangia embedded in the leaf base.
  • Salvinia & Azolla: Aquatic ferns exhibiting highly reduced and specialized sporocarps containing both micro- and megasporangia.

Seed Habit in Pteridophytes

The seed habit represents a further evolutionary advancement, characterized by the retention of the megaspore within the megasporangium (nucellus) after fertilization. This retention provides protection to the developing embryo and facilitates dispersal. While true seeds, as found in gymnosperms and angiosperms, are not present in pteridophytes, certain features foreshadow the seed habit.

  • Retention of Megaspore: In heterosporous pteridophytes, the megaspore is not released at the time of spore formation. It remains within the megasporangium.
  • Development of Embryo within Megaspore: After fertilization, the zygote develops into an embryo within the megaspore.
  • Protective Covering: The megasporangial wall provides a protective covering for the developing embryo.

Examples:

  • Selaginella: The megaspore remains within the megasporangium for a considerable period, allowing for embryo development.
  • Isoetes: Similar to *Selaginella*, the megaspore is retained within the nucellus.

Comparison with Homospory

Feature Homospory Heterospory
Spore Type Single type of spore Two types of spores (micro & mega)
Gametophyte Single type of gametophyte Two types of gametophytes (male & female)
Fertilization Often reliant on water for sperm transport More efficient sperm delivery
Evolutionary Significance Represents an ancestral condition A crucial step towards seed evolution

Conclusion

Heterospory and the incipient seed habit in pteridophytes represent critical evolutionary milestones in plant life. These adaptations, particularly evident in genera like *Selaginella* and *Isoetes*, demonstrate a shift towards greater reproductive efficiency and protection of the developing embryo. While not true seeds, the retention of the megaspore and embryo within the megasporangium laid the groundwork for the evolution of seeds in gymnosperms and angiosperms, ultimately shaping the terrestrial plant landscape. Further research into the genetic and developmental mechanisms underlying these transitions will continue to illuminate the evolutionary history of plants.

Answer Length

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Additional Resources

Key Definitions

Heterospory
The production of two distinct types of spores – microspores and megaspores – by a plant. This is a key evolutionary adaptation in certain pteridophytes and is considered a precursor to the seed habit.
Seed Habit
The evolutionary development of retaining the megaspore within the megasporangium, providing protection and nourishment to the developing embryo. This is a key characteristic of seed plants (gymnosperms and angiosperms).

Key Statistics

Approximately 20% of pteridophyte species are estimated to be heterosporous (Smith et al., 2006).

Source: Smith, A. R., et al. (2006). A revised classification of extant ferns and lycophytes. *Taxon*, 55(4), 705-728.

The earliest fossil evidence of plants exhibiting features suggestive of a seed habit dates back to the late Devonian period, approximately 360 million years ago (Kenrick & Crane, 1997).

Source: Kenrick, P., & Crane, P. R. (1997). The origin and evolution of seeds. *Nature*, 389(6653), 863-869.

Examples

Aquatic Heterospory in *Salvinia*

*Salvinia*, a free-floating aquatic fern, exhibits a unique form of heterospory. Its sporocarps contain both microsporangia and megasporangia, and the megaspore develops into a highly reduced female gametophyte within the sporocarp, demonstrating a clear precursor to seed development.

Frequently Asked Questions

▶What is the significance of the megaspore wall?

The megaspore wall provides a protective barrier against desiccation, mechanical damage, and pathogens, enhancing the survival of the developing female gametophyte and embryo. It's a crucial adaptation for terrestrialization and a precursor to the seed coat.

Topics Covered

BotanyPlant KingdomPteridophytesEvolutionReproduction
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