How far does Selaginella one of the few living members of lycopodiales (pteridophytes) fall short of seed habi

NCERT Exemplar · Q19

Q. How far does Selaginella one of the few living members of lycopodiales (pteridophytes) fall short of seed habit.

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Concept understanding — Heterospory In Pteridophytes

Heterospory in Pteridophytes

Imagine a plant that reproduces by spores. Most ferns you've seen release thousands of identical dust-like spores — each one is the same size, same shape, and has the same potential to grow into a tiny new plant. That's homospory (one kind of spore). Now picture a plant that makes two distinctly different types of spores: one tiny and numerous, the other large and few. That's heterospory — and it's a quiet revolution in plant evolution.

The Intuition: Why Two Kinds of Spores?

Think about what a spore needs to do: it must survive, germinate, and produce sex cells. A small spore is cheap to make — you can produce millions — but it carries very little food. A large spore is expensive, but it comes packed with nutrients, giving the next stage a head start. In a challenging environment, the large spore's offspring has a much better chance of surviving.

Heterospory is nature's way of specialising: the microspore (small, many) becomes the male, and the megaspore (large, few) becomes the female. This division of labour is the first step toward the seed.

The Precise Statement

Important

Heterospory is the production of two morphologically different types of spores by the same plant: microspores (small, numerous) and megaspores (large, few in number). It occurs in certain pteridophytes like Selaginella and Salvinia, and in all seed plants.

The microspore germinates to form a male gametophyte (which produces sperm). The megaspore germinates to form a female gametophyte (which produces the egg). Crucially, the female gametophyte remains retained inside the megaspore wall — it never becomes free-living. This retention is the key evolutionary link to the seed habit.

How It Works in Selaginella (A Living Example)

In Selaginella (spike moss), a single cone-like structure called a strobilus bears two kinds of sporangia:

Microsporangia — produce many tiny microspores.
Megasporangia — produce usually just four large megaspores.

When released, the microspore germinates into a tiny male gametophyte that releases flagellated sperm. The megaspore germinates inside its own wall, producing a female gametophyte with archegonia (egg-bearing structures). The sperm swims to the megaspore, fertilises the egg, and the zygote develops into a new sporophyte — all while still nourished by the megaspore's food reserves.

Note

In homosporous pteridophytes, the gametophyte is free-living and independent. In heterosporous ones, the female gametophyte is endosporic — it develops inside the spore wall, dependent on the stored food. This is a major shift.

Why It Matters: The Precursor to the Seed Habit

Heterospory alone is not the seed habit — but it sets the stage. A seed requires three things:

Heterospory (microspores and megaspores)
Retention of the megaspore within the megasporangium (it is not shed)
Development of the embryo while still attached to the parent plant …

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Selaginella shows the retention and embryo-development pattern of a seed but lacks the protective covering that defines a true seed.

Selaginella is one of the few living pteridophytes that is heterosporous, producing both megaspores and microspores. In heterosporous plants generally, the megaspore germinates into a female gametophyte that is held on the parent sporophyte, and after fertilisation the zygote develops into a young embryo while still attached to and nourished by the parent -- this pattern is regarded as a forerunner of the seed habit.

Selaginella follows exactly this pattern: its female gametophyte and the embryo that later forms from the zygote both develop while retained on the parent plant, rather than being released as an independent, free-living gametophyte the way a homosporous fern's prothallus would be.

Note

What is missing, compared with a true seed plant, is the further step seen in gymnosperms and angiosperms -- the megasporangium wrapped in protective integuments that later mature into a hard, dispersible seed coat around the embryo. Selaginella's retained megaspore and embryo are not built up with that kind of protective seed-coat structure. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

TG EAPCET 2021Set ap-2021-08-10-AN1 markMCQ
$Q. Assertion (A): The development of zygote into young embryo with in female gametophyte indicate the precursor for the seed habit. Reason (R): In pteridophytes, the gametophyte requires cool, damp, shady place to grow. The correct option among the following is (A) (A) is true, (R) is true and (R) is the correct explanation for (A) (B) (A) is true, (R) is true but (R) is not the correct explanation for (A) (C) (A) is true but (R) is false (D) (A) is false but (R) is true$
›Reveal solutionSolution
Retention of the megaspore, with the zygote maturing into an embryo inside the female gametophyte, really is the precursor of the seed habit (A true), and pteridophyte gametophytes really do need cool damp shade (R true) — but the second fact does not explain the first. Option (B).

The concept first: how the seed habit began

A seed is, structurally, an embryo + food reserve + protective coat, all developed while attached to the parent plant. Evolution did not arrive at this in one leap. Track the steps in pteridophytes:

Homospory — one kind of spore, one kind of bisexual gametophyte, shed and free-living (most ferns).

Heterospory — two spore sizes appear: small microspores (male) and large megaspores (female). Seen in Selaginella and Salvinia.

Retention of the megaspore. The megaspore is not shed; it stays on the parent sporophyte, inside the megasporangium.

The female gametophyte develops inside the retained megaspore. After fertilisation, the zygote develops into a young embryo while still enclosed within the female gametophyte, still on the parent plant.

Step 4 is the crucial innovation. Nourishing and protecting the young embryo on the parent is exactly what a seed does. So the NCERT wording: this event is a precursor to the seed habit, considered an important step in evolution.

Step-by-step

Assertion: "The development of zygote into a young embryo within the female gametophyte indicates the precursor for the seed habit." This is the textbook statement of the point above. A is TRUE.

Reason: "In pteridophytes, the gametophyte requires cool, damp, shady places to grow." Also TRUE — the pteridophyte prothallus is small, thin-walled, without cuticle, and (crucially) needs a film of water so that antherozoids can swim to the archegonium. …
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TG EAPCET 2021Set ap-2021-08-10-FN1 markMCQ
$Q. Assertion (A): The development of zygote into young embryo with in female gametophyte indicates the precursor for the seed habit. Reason (R): In pteridophytes, the gametophyte requires cool, damp, shady place to grow. The correct option among the following is: (A) (A) is true, (R) is true and (R) is the correct explanation for (A) (B) (A) is true, (R) is true but (R) is not the correct explanation for (A) (C) (A) is true but (R) is false (D) (A) is false but (R) is true$
›Reveal solutionSolution
The assertion is true because the retention and development of the zygote within the female gametophyte is a key evolutionary step toward the seed habit. The reason is also true (pteridophyte gametophytes need moist, shady conditions), but it does not explain the assertion — it describes a separate ecological requirement. So both are true, but the reason is not the correct explanation.

Concept and Intuition

The seed habit is a major evolutionary innovation in land plants. In seed plants (gymnosperms and angiosperms), the embryo develops while still attached to and nourished by the parent sporophyte — this protects the vulnerable young sporophyte. The precursor to this is seen in some pteridophytes (like Selaginella and Salvinia) where the zygote develops into an embryo inside the female gametophyte, which itself remains on the parent sporophyte. This retention is a step toward the seed.

The reason given is a separate fact: pteridophyte gametophytes (the free-living, independent stage) are delicate and require moist, shaded habitats for fertilization and growth. This fact is true but does not explain why zygote retention is a precursor to seeds.

Step-by-step reasoning

Evaluate Assertion (A):

In some pteridophytes (e.g., heterosporous ones like Selaginella), the female gametophyte is retained within the megaspore wall, and the zygote develops into an embryo while still inside this gametophyte. This is indeed considered an evolutionary precursor to the seed habit because seeds are defined by the retention of the female gametophyte and the developing embryo within the parent sporophyte.

→ Assertion (A) is true.

Evaluate Reason (R):

Pteridophytes (ferns, horsetails, lycophytes) have free-living, independent gametophytes (prothalli) that lack a cuticle and vascular tissue. They require a cool, damp, shady environment for water-dependent fertilization and to prevent desiccation.

→ Reason (R) is true.

Check if (R) explains (A): …
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Q.How far does Selaginella one of the few living members of lycopodiales (pteridophytes) fall short of seed habit.

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