Q.21 Among the following pteridophytes, the initial steps in evolutionary trend of seed development can be best seen in
Selaginella best shows the initial evolutionary steps toward seed development among pteridophytes. This multiple-choice question tests knowledge of heterospory and seed habit precursors in these vascular, spore-producing plants. The correct answer is (2) Selaginella.
Option Analysis
Lycopodium (1)
Lycopodium is homosporous, producing only one type of spore that germinates into a bisexual gametophyte outside the sporangium. It lacks heterospory, the first key step for seed evolution, so no initial trends appear here.
Selaginella (2)
Selaginella exhibits heterospory with microspores and megaspores; megaspores often retain and germinate inside the megasporangium, forming a female gametophyte nourished by the sporophyte. These match early seed habit steps: heterospory, megaspore retention, and partial embryo protection, making it the best example.
Isoetes (3)
Isoetes is heterosporous like Selaginella, with megaspores retained in sporangia on leaves, showing some seed-like features. However, Selaginella demonstrates more advanced megaspore reduction (e.g., one functional megaspore in some species) and in situ development.
Rhynia (4)
Rhynia, a primitive rhyniophyte (Cooksonia-like), is homosporous with terminal sporangia releasing all spores externally. It shows no heterospory or retention, representing early pteridophyte reproduction without seed trends.
Introduction to Evolutionary Trend of Seed Development Pteridophytes
Pteridophytes mark a key transition in plant evolution, with evolutionary trend of seed development pteridophytes visible through heterospory—the production of microspores (male) and megaspores (female). This leads to seed habit precursors like megaspore retention inside sporangia, embryo nourishment by the parent, and reduced gametophytes, bridging free-sporing pteridophytes to seed-producing gymnosperms.
Key Features in Selaginella
Selaginella leads with advanced heterospory: four megaspores form per megasporangium, but often only one functions and germinates in situ (e.g., S. rupestris). The female gametophyte develops archegonia inside, with embryo formation before shedding—closest to true seeds among options.
Comparison Table: Heterospory and Seed Traits
| Pteridophyte | Homosporous/Heterosporous | Megaspore Retention | Seed Habit Closeness |
|---|---|---|---|
| Lycopodium | Homosporous | None | None |
| Selaginella | Heterosporous | Yes, in situ germination | Highest |
| Isoetes | Heterosporous | Partial | Moderate |
| Rhynia | Homosporous | None | None |
Why Not Other Pteridophytes?
Lycopodium and Rhynia stick to primitive homospory with external gametophytes. Isoetes advances heterospory but lags in megaspore reduction compared to Selaginella’s near-seed state.
Exam Relevance
For competitive exams like NEET or GATE Life Sciences, recognize Selaginella’s evolutionary trend of seed development pteridophytes as the model—heterospory evolved here first among options, enabling seed origins.
