Genera like Selaginella and Salvinia which produce two kinds of spores, macro (large) and micro (small) spores, are known as heterosporous. The megaspores and microspores germinate and give rise to female and male gametophytes, respectively. The female gametophytes in these plants are retained on the parent sporophytes for variable periods. The development of the zygotes into young embryos take place within the female gametophytes. This event is a precursor to the seed habit considered an important step in evolution. Assertion – In gymnosperms, the male and female gametophytes depend on the sporophyte.

Assertion & Reason: Gametophyte Dependence in Gymnosperms

Assertion (A):

In gymnosperms, the male and female gametophytes depend on the sporophyte.

Reason (R):

Gymnosperms are universally heterosporous.

Explanation:

1. What is Needed for an Independent Gametophyte?

For a gametophyte to be independent, it should:

• Be able to grow outside the sporophyte’s body.
• Have its own nutrition source (like photosynthesis or absorbing nutrients from soil/water).
• Survive on its own without being enclosed inside sporophytic tissues.

2. What Happens in Heterospory?

In heterospory, two types of spores are produced:

• Microspores → Develop into male gametophytes (pollen grains).
• Megaspores → Develop into female gametophytes (inside the ovule).

Key Point: In heterospory, the megaspore never leaves the sporophyte!

The female gametophyte (inside megaspore) stays within the ovule, which remains attached to the sporophyte. It does not get released into the environment to grow independently.

3. How Does This Prevent Gametophyte Independence?

• Since the megaspore remains inside the ovule, it cannot develop on its own in external conditions (like mosses and ferns do).
• It is nourished by the sporophyte through the ovular tissues.
• The male gametophyte (pollen grain) is not free-living either—it needs to reach the ovule (through pollination) to complete its function.

Thus, heterospory ensures that gametophytes remain enclosed within sporophytic tissues, making them fully dependent on the sporophyte for survival and reproduction.

4. How is This Different from Homospory?

• In homosporous plants (like mosses and ferns), a single type of spore is produced.
• This spore is released into the environment and grows into a free-living, independent gametophyte that can photosynthesize and survive on its own.
• But in heterospory, gametophytes remain inside sporophytic structures, making them dependent.

5. Conclusion: Why R Explains A?

• ✔ Gymnosperms are heterosporous (R).
• ✔ Heterospory prevents the gametophyte from being released into the environment, forcing it to depend on the sporophyte for nutrition.
• ✔ This is why, in gymnosperms, male and female gametophytes depend on the sporophyte (A).

Thus, heterospory is the reason why gametophytes in gymnosperms cannot live independently—making R the correct explanation of A. ✅