Q.47 Given below are two statements : One is labelled as Assertion (A) and the other is labelled as
Reason (R).
Assertion (A) :In pteridophytes, genera like Se/agine//a and Sa/vinia produces heterospores.
Reasons (R) :Hetrospory in pteridophytes is a precursor to the seed habit. which is an
important step in evolution
In the light Of the above statements, choose the most appropriate answer from the options given
below :
(1)Both (A) and (R) are correct and (R) is the correct explanation of (A)
(2)Both (A) and (R) are correct but (R) is NOT the correct explanation of (A)
(3)(A) is correct but (R) is not correct
(4)(A) is not correct but (R) is correct
Both Assertion (A) and Reason (R) are correct, and (R) correctly explains (A)
Selaginella and Salvinia exhibit heterospory in pteridophytes, and this condition represents a crucial evolutionary precursor to the seed habit, making option (1) correct.
Introduction
Heterospory in pteridophytes like Selaginella and Salvinia marks a critical evolutionary transition from homospory to seed habit. These genera produce microspores and megaspores, with female gametophytes often retained within sporangia—foreshadowing seed development in higher plants.
Statement Analysis
Assertion (A): Correct. Selaginella (lycopsid) and Salvinia (water fern, Marsileaceae) are classic heterosporous pteridophytes producing two spore types:
-
Selaginella: Microsporangia (microspores → male gametophytes), megasporangia (4 megaspores → female gametophyte retained inside sporangium)
-
Salvinia: Similar dimorphism; sporocarps contain both spore types
Reason (R): Correct. Heterospory is prerequisite for seed evolution:
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Differentiation: Microspores → male; megaspores → female gametophytes
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Retention: Female gametophyte develops within megasporangium
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Dependency: Embryo nourished by parent sporophyte
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Protection: Precursor to integuments
Relationship: (R) explains why heterospory appeared—evolutionary pressure for seed habit.
Option Evaluation
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(1) Both correct, R explains A: Correct answer
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(2) Both correct, R doesn’t explain: Incorrect—seed habit directly links heterospory
-
(3) A correct, R incorrect: Incorrect—R factually true
-
(4) A incorrect, R correct: Incorrect—both Selaginella/Salvinia heterosporous
Evolutionary Progression
Homospory (most ferns) → Heterospory (Selaginella) → Seed habit (Lyginopteris) → Seeds (gymnosperms)
Heterospory advantages:
| Feature | Homosporous | Heterosporous |
|---|---|---|
| Spore types | Isospores | Micro + mega |
| Gametophyte | Independent | Female retained |
| Nutrient allocation | Equal | Female prioritized |
| Water dependence | High | Reduced |
Selaginella seed habit:
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1 functional megaspore/4
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Endosporic gametophyte
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Embryo dependent on sporophyte
Exam Relevance
GATE Life Sciences: Option 1 signature for heterospory-seed evolution questions. Key genera: Selaginella (lycopod), Salvinia/Marsilea (water ferns). Mnemonic: “SELaginella SEED habit SELection.”
Distinction: Heterospory = spore dimorphism; seed habit = gametophyte retention + dependency.
Paleobotanical Evidence
Devonian fossils show homosporous → heterosporous transition; Lepidodendrales, Calamites exhibit advanced heterospory but no true seeds—evolutionary “missing link” confirmed.
