Step 1: Understanding the Concept:
Algae are primitive, chlorophyll-bearing, autotrophic organisms classified primarily by their photosynthetic pigments.
The three main classes are Chlorophyceae (green), Phaeophyceae (brown), and Rhodophyceae (red).
Brown algae (Phaeophyceae) are predominantly marine and range from simple branched filaments to massive kelps that can reach 100 meters in length.
Because they live in water, large algae face a challenge: they need to reach sunlight for photosynthesis, but their heavy bodies naturally sink.
To solve this, many brown algae have evolved "air bladders" or "pneumatocysts."
These are gas-filled sacs that act like life jackets, keeping the photosynthetic fronds floating near the water's surface where light intensity is highest.
Step 2: Detailed Explanation:
Let's analyze each of the biological choices provided:
- Polysiphonia (Option A): This is a member of Rhodophyceae (red algae). It has a complex, multi-axial filamentous structure. It does not possess air bladders and typically lives in deeper waters where red pigments are more efficient.
- Fucus (Option B): This is a classic example of Phaeophyceae (brown algae). Commonly known as "rockweed," it grows on rocky shorelines. It is easily identifiable by its flat, dichotomously branched thallus which features prominent, paired air bladders. These bladders are essential for keeping the plant upright when the tide comes in.
- Gelidium (Option C): This is another red alga (Rhodophyceae). It is economically important as a source of agar-agar but lacks air bladders.
- Porphyra (Option D): Also a red alga, often used to make "nori." It has a thin, leafy thallus and does not require buoyancy organs as it often grows in intertidal zones where it is supported by the ground or water surface tension.
Conclusion: Among the list, only {Fucus} is a brown alga with the specified structural adaptation of air bladders. Another famous example often mentioned in textbooks is {Sargassum}, which forms vast floating mats in the ocean.
Step 3: Final Answer:
The characteristic of possessing air bladders for buoyancy in a marine brown alga identifies the plant as {Fucus}.
Therefore, option (B) is the correct choice.