Model Answer
0 min readIntroduction
*Psilotum* is a peculiar vascular plant often referred to as the ‘whisk fern’ due to its simple, primitive morphology. It represents a relict genus, offering insights into the early evolution of vascular plants. Belonging to the Psilotaceae family, *Psilotum* exhibits a unique plant body lacking true roots and leaves, features that have led botanists to consider it a living fossil, closely related to the extinct group Psilotales which flourished during the Devonian and Carboniferous periods. Understanding its structure and primitive traits is crucial for reconstructing the evolutionary history of land plants.
Plant Body of *Psilotum*
The plant body of *Psilotum* is dichotomously branched and is differentiated into three main parts: rhizome, aerial branches, and appendages.
1. Rhizome
The rhizome is a subterranean, horizontally growing stem. It is extensively branched and covered with scales. It anchors the plant and stores food. Small, peg-like structures called rhizoids arise from the lower surface of the rhizome, performing a function analogous to roots, but are not true roots as they lack vascular tissue.
2. Aerial Branches
The aerial branches arise from the rhizome and exhibit dichotomous branching – meaning they split into two equal branches at each fork. These branches are green and photosynthetic, carrying out the primary function of food production. They are cylindrical and lack leaves.
3. Appendages
Small, scale-like appendages are present on the aerial branches. These appendages are simple, sessile (without a stalk), and lack vascular tissue. Their function is debated, but they may play a role in photosynthesis or protection. They are not considered true leaves.
Primitive Characters of *Psilotum* and its Relation to Psilotales
*Psilotum* exhibits several primitive characters that strongly suggest its close relationship to the extinct Psilotales. These characters are reminiscent of early vascular plants and provide valuable clues about plant evolution.
- Lack of True Roots: *Psilotum* lacks true roots, relying instead on rhizoids for anchorage and absorption. This is a primitive feature found in early vascular plants.
- Absence of Leaves: The absence of true leaves is another primitive characteristic. The appendages are structurally simple and lack the complex vascularization of leaves.
- Simple Sporangia: The sporangia (spore-producing structures) of *Psilotum* are simple, spherical structures borne on the aerial branches. They lack a stalk and a specialized mechanism for spore dispersal, resembling those found in early vascular plants like the Psilotales.
- Dichotomous Branching: The dichotomous branching pattern is a primitive mode of branching observed in many early vascular plants.
- Vascular Tissue Organization: The vascular tissue (xylem and phloem) in *Psilotum* is relatively simple in organization, lacking the complex secondary growth found in more advanced vascular plants. The protostele type of stele is observed.
- Synangia Formation: Sporangia are fused to form synangia, a characteristic feature of Psilotales.
Fossil evidence of Psilotales, dating back to the Devonian period, reveals plants with similar morphological features to *Psilotum*, further supporting their close evolutionary relationship. The simplicity of *Psilotum*'s structure is considered a result of secondary simplification, meaning it may have evolved from a more complex ancestor, losing certain features over time.
Conclusion
In conclusion, *Psilotum* presents a fascinating example of a living fossil, showcasing a plant body characterized by a rhizome, aerial branches, and appendages. Its primitive features – the absence of true roots and leaves, simple sporangia, and dichotomous branching – strongly indicate its close evolutionary relationship to the extinct Psilotales, providing valuable insights into the early evolution of vascular plants and their adaptation to terrestrial environments. Further research into its genome and developmental biology will continue to refine our understanding of its phylogenetic position.
Answer Length
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