The bacteria that is found in the root nodules of leguminous plants is

1. Analyze the question The question asks which bacterium lives inside the root nodules of leguminous plants (plants like beans, groundnut, cowpea, and soybean).

2. Explain the concept Leguminous plants form a symbiotic (mutualistic) relationship with nitrogen-fixing bacteria:

• The bacteria enter the root hairs and cause the formation of small swellings called root nodules.
• Inside the nodules, the bacteria convert atmospheric nitrogen (N₂) into ammonia/nitrates that the plant can use. This is nitrogen fixation.
• In return, the plant gives the bacteria food (carbohydrates) and shelter.

3. Evaluate each option

• A. Clostridium: a free-living, anaerobic nitrogen-fixing bacterium in the soil. It is not found in root nodules.
• B. Rhizobium: the symbiotic bacterium that lives in the root nodules of legumes. ✔
• C. Nostoc: a blue-green alga (cyanobacterium) that fixes nitrogen, and is free-living or found in water and moist soil.
• D. Azotobacter: a free-living, aerobic nitrogen-fixing bacterium in the soil, not in nodules.

4. Answer The correct option is B. Rhizobium.

5. Tip Remember: "Rhizo" = root. Rhizobium = the root-nodule bacterium. Also remember that Azotobacter and Clostridium are free-living nitrogen fixers, so they do not live in nodules. JAMB often tests this contrast between symbiotic and free-living nitrogen fixers.