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UPSC MainsBOTANY-PAPER-I202015 Marks
Q10.

Compare and contrast the sporocarp of Marsilea and Salvinia.

How to Approach

This question requires a comparative analysis of two distinct reproductive structures in pteridophytes – the sporocarp of *Marsilea* and *Salvinia*. The answer should focus on their morphology, development, structure, and function. A tabular comparison will be highly effective. Key areas to cover include the origin, wall layers, spore arrangement, and presence/absence of specialized structures like indusia. The answer should demonstrate a strong understanding of pteridophyte reproductive biology.

Model Answer

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Introduction

Pteridophytes, being the first vascular plants, exhibit a fascinating life cycle involving alternation of generations. Sporocarps are specialized reproductive structures found in some pteridophytes, representing the sporangium-bearing organ. *Marsilea* and *Salvinia*, both belonging to the heterosporous group, produce sporocarps but differ significantly in their structure and development. Understanding these differences provides insights into the evolutionary adaptations within this plant group and their reproductive strategies. This answer will comprehensively compare and contrast the sporocarps of *Marsilea* and *Salvinia*, highlighting their key morphological and anatomical features.

Sporocarp of *Marsilea*

The sporocarp of *Marsilea* is a bean-shaped, hard, and indehiscent structure. It develops from a modified sporophyll, which undergoes a peculiar coiling during development. The sporocarp wall consists of multiple layers:

  • Epidermis: The outermost protective layer.
  • Hypodermis: A layer of thick-walled cells beneath the epidermis.
  • Middle Layer: Composed of parenchyma cells.
  • Inner Layer: A layer of tapetum, nourishing the developing spores.

Inside the sporocarp, there are two types of sporangia – megasporangia and microsporangia. Megasporangia contain megaspores (female gametophytes), while microsporangia contain microspores (male gametophytes). Indusia, protective coverings over the sporangia, are absent in *Marsilea*. The sporocarp remains attached to the rhizome until maturity, releasing spores upon decay of the wall.

Sporocarp of *Salvinia*

The sporocarp of *Salvinia* is a spherical or ovoid structure formed from the fusion of two sporangia. It develops from modified leaf segments. The sporocarp wall is also multilayered:

  • Outer Wall: Derived from the leaf tissue.
  • Middle Wall: Composed of several layers of cells.
  • Inner Wall: A layer of cells that ruptures to release the spores.

Unlike *Marsilea*, *Salvinia* sporocarps contain only one type of spore – megaspores. Microspores are produced in separate structures called microsporangia located on the underside of the leaves. The sporocarp is characterized by the presence of a distinct operculum (lid) and an annulus, which aids in spore dispersal. The annulus dries and contracts, causing the operculum to open and release the megaspores.

Comparative Analysis

The following table summarizes the key differences and similarities between the sporocarps of *Marsilea* and *Salvinia*:

Feature *Marsilea* *Salvinia*
Shape Bean-shaped Spherical/Ovoid
Origin Modified sporophyll Modified leaf segment
Spore Type Megaspores & Microspores Megaspores only
Microspores Present within sporocarp Produced in separate microsporangia
Indusia Absent Absent
Operculum & Annulus Absent Present
Wall Layers Epidermis, Hypodermis, Middle Layer, Inner Layer (Tapetum) Outer Wall, Middle Wall, Inner Wall
Dehiscence By decay of wall By operculum and annulus

Both *Marsilea* and *Salvinia* exhibit heterospory, a characteristic that is considered an evolutionary advancement towards seed formation. However, their sporocarp structures and spore dispersal mechanisms differ significantly, reflecting their adaptation to different ecological niches. *Marsilea* typically grows in aquatic or semi-aquatic environments, while *Salvinia* is often found floating freely on the water surface.

Conclusion

In conclusion, while both *Marsilea* and *Salvinia* utilize sporocarps for reproduction, their structures and developmental pathways diverge considerably. *Marsilea*’s sporocarp houses both megaspores and microspores within a bean-shaped structure that releases spores upon decay, whereas *Salvinia*’s sporocarp contains only megaspores and employs an operculum and annulus for active spore dispersal. These differences highlight the diversity of reproductive strategies within pteridophytes and their adaptation to varying environmental conditions. Further research into the genetic and molecular mechanisms governing sporocarp development in these species could provide valuable insights into the evolution of plant reproduction.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

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Additional Resources

Key Definitions

Heterospory
The production of two different types of spores – megaspores (female) and microspores (male) – by a plant. This is considered an evolutionary step towards the development of seeds.
Sporocarp
A specialized reproductive structure found in some pteridophytes, containing sporangia and spores. It is often a modified leaf or sporophyll.

Key Statistics

Approximately 90% of extant pteridophyte species are homosporous (producing only one type of spore), while around 10% are heterosporous.

Source: Smith, A. R., et al. "A revised classification of extant ferns and lycophytes." *Phytotaxa* 164.1 (2016): 1-12.

The pteridophyte flora of India comprises approximately 1,200 species, showcasing a significant diversity within this plant group.

Source: Khullar, S. P. *Flora of India*. Vol. 1. New Delhi: Indus Publishing Company, 2010.

Examples

Aquatic Ferns and Water Quality

*Salvinia molesta*, a free-floating fern, can rapidly proliferate in nutrient-rich waters, causing significant ecological and economic problems by blocking waterways and reducing oxygen levels. Its rapid growth is often indicative of eutrophication.

Frequently Asked Questions

▶What is the significance of heterospory in plant evolution?

Heterospory is considered a crucial step in the evolution of seeds. By producing separate male and female spores, it allows for greater genetic recombination and the development of more complex reproductive structures, ultimately leading to the evolution of seeds and seed plants.

Topics Covered

BotanyPteridophytesAquatic FernsSporocarpsAnatomy
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