Problem 136 - Objective Biology for NEET

Question

Majority of the pteridophytes are I. Homosporous and produce only one kind of spores. II. Aquatic in nature. III. Heterosporus and produce small and large spores. IV. Restricted to narrow geographical zone because of specific requirement of water. (a) I and II (b) II and III (c) III and IV (d) IV and I

Step-by-Step Solution

Answer

The correct answer is (d) IV and I. A majority of pteridophytes are homosporous and produce only one kind of spore. They are also restricted to a narrow geographical zone because they require specific amounts of water.

Step 1: Evaluate Statement I

Statement I is true. Homosporous pteridophytes produce only one kind of spore.

Step 2: Evaluate Statement II

Statement II is false. While pteridophytes do need a moist environment to survive, they are not typically aquatic.

Step 3: Evaluate Statement III

Statement III is only partly true. While some pteridophytes are heterosporous and produce small and large spores, not all pteridophytes are like this. It is more accurate to say a majority of pteridophytes are homosporous.

Step 4: Evaluate Statement IV

Statement IV is true. Pteridophytes are restricted to a narrow geographical zone because they have a specific requirement of water.

Key Concepts

Homosporous

Most pteridophytes are homosporous, meaning they produce only one type of spore. This simplifies their reproductive process as these single spores contain both male and female reproductive organs. This way, when these spores germinate, they develop into a gametophyte that can produce both sperm and eggs.

• These gametophytes are typically similar in structure and do not show much variation.
• The unisexual nature of homosporous spores makes them more reliant on external conditions for successful reproduction.

Heterosporous

In contrast to homosporous pteridophytes, heterosporous ones produce two types of spores. This adaptation allows for a more specialized reproductive strategy.

• Microspores develop into male gametophytes, while megaspores develop into female gametophytes.
• Having distinct types of spores helps in the protection and sustenance of the delicate reproductive cells.

An example of heterosporous pteridophytes can be found in genera such as Selaginella and Marsilea, where you see an evolutionary advancement towards seed plants.

Geographical distribution

Pteridophytes have a limited geographical distribution due to their specific environmental needs. They are often found in regions where humidity and shade are abundant, such as tropical and temperate forests.

• These conditions help maintain the moisture level necessary for their reproduction and growth.
• While present worldwide, their density and diversity are significantly higher in areas with consistent precipitation.

These plants serve as an essential part of their ecosystem, offering habitat and playing a part in nutrient cycles.

Moisture requirement

Moisture is perhaps the most critical environmental factor for pteridophyte survival. As they do not have seeds, their sperms need water to swim to the eggs for fertilization.

• This dependency restricts them to habitats with sufficient rainfall or high humidity.
• In drier areas, pteridophytes might depend on microhabitats that retain moisture, such as crevices or near water bodies.

This requirement for moist conditions is a reason why they are often found in shaded and damp forest floors, where water is more available.