Giving suitable reasons, describe inter-alia, evolutionary status of Psilotum, Selaginella and Marsilea.

Psilotum: A Relic of Early Vascular Plant Evolution

Psilotum, often referred to as whisk fern, is a peculiar plant that has long fascinated botanists due to its simple morphology. It lacks true roots and leaves, possessing only dichotomously branching stems with scale-like appendages. Its evolutionary status was initially debated, with some considering it a primitive vascular plant, even a direct ancestor of ferns. However, molecular phylogenetic studies have revealed a more nuanced picture.

• Evolutionary Status: Psilotum is now considered a highly reduced fern ally, belonging to the Psilotales order. It is not a primitive ancestor but rather a descendant that has undergone significant secondary simplification.
• Primitive Features: The absence of true roots and leaves, along with its simple vascular tissue, suggests a loss of complexity rather than a primitive state.
• Advanced Features: Its sporangia are borne on short stalks, and it exhibits a unique synangium structure, indicating a derived condition. The presence of underground rhizomes for perennation is also a more advanced feature.
• Evidence: Molecular data places Psilotum firmly within the ferns and fern allies (monilophytes), demonstrating its evolutionary relationship to more complex ferns.

Selaginella: A Representative of Early Lycophytes

Selaginella, commonly known as spikemoss, is a genus of lycophytes characterized by its small, scale-like leaves arranged spirally or oppositely on the stem. It differs from other lycophytes like Lycopodium in having ligules (small flaps of tissue) on the upper surface of the leaves.

• Evolutionary Status: Selaginella represents a more advanced group of lycophytes compared to early forms like Baragwanathia. It is considered a crucial link in understanding the evolution of heterospory.
• Primitive Features: Its vascular tissue is relatively simple, and it reproduces via spores. The microphylls (small, single-veined leaves) are considered a primitive feature of lycophytes.
• Advanced Features: Selaginella is renowned for exhibiting heterospory – the production of two distinct types of spores (microspores and megaspores). This is a significant evolutionary step towards the development of seeds. The presence of ligules is also a derived character.
• Significance: The evolution of heterospory in Selaginella is considered a precursor to the evolution of seeds, as it allows for the development of specialized male and female gametophytes.

Marsilea: A Water Fern with Seed-Like Characteristics

Marsilea, a genus of aquatic ferns, is characterized by its four-lobed leaves that resemble clover leaves. It is a heterosporous plant, and its sporocarps (spore-containing structures) are unique and resemble seeds in their protective function and dispersal mechanisms.

• Evolutionary Status: Marsilea is a relatively advanced fern, belonging to the Salviniales order. It represents a significant step in the evolution of vascular plants, exhibiting features that foreshadow the development of seeds.
• Primitive Features: It reproduces via spores and requires water for fertilization, indicating a dependence on moist environments.
• Advanced Features: Marsilea exhibits pronounced heterospory, with highly reduced male gametophytes contained within the microspores and female gametophytes developing within the megaspores. Its sporocarps provide protection to the developing spores and aid in dispersal, resembling seeds in function. The sporocarp wall has multiple layers, offering robust protection.
• Adaptations: The sporocarp’s ability to withstand desiccation and disperse via water currents demonstrates adaptations that are analogous to those found in seed plants.

Comparative Table of Evolutionary Status

Plant	Primitive Features	Advanced Features	Evolutionary Significance
Psilotum	Lack of true roots & leaves, simple vascular tissue	Synangium structure, rhizomes	Represents secondary simplification within ferns; insights into fern evolution.
Selaginella	Microphylls, spore reproduction	Heterospory, ligules	Crucial link in the evolution of heterospory; precursor to seed evolution.
Marsilea	Spore reproduction, water dependence	Pronounced heterospory, sporocarps resembling seeds	Demonstrates adaptations analogous to seed plants; provides insights into seed evolution.