The part of a leguminous plant where bacteria like Azotobacter can be found is

Analyze the question: It asks where nitrogen-fixing bacteria are found in a leguminous plant (beans, groundnut, cowpea, etc.).

Concept: Legumes live in a symbiotic relationship with nitrogen-fixing bacteria. The bacteria infect the roots and cause small swellings called root nodules. Inside these nodules, the bacteria convert atmospheric nitrogen (N₂) into ammonia/nitrates that the plant can use. In return, the plant supplies the bacteria with carbohydrates and shelter.

A note on the name: The classic root-nodule bacterium in legumes is Rhizobium. Azotobacter is actually a free-living nitrogen-fixer in the soil. However, the question is set about leguminous plants and asks for the part of the plant associated with nitrogen-fixing bacteria, so the intended answer is the root nodules.

Checking the options:

• A. Nodes of the stem: Nodes are where leaves and branches attach. No nitrogen-fixing bacteria live there.
• B. Internodes: These are just the stem sections between nodes, so this is incorrect.
• C. Root nodules: Correct. This is the site of nitrogen fixation in legumes.
• D. Spongy mesophyll: This is leaf tissue involved in gas exchange and photosynthesis, so this is incorrect.

Answer: C. in the root nodules.

Exam tip: Remember "Legume = Root nodule = Rhizobium = Nitrogen fixation." Whenever a question mentions nitrogen-fixing bacteria in legumes, think of the roots. Also remember that Nitrosomonas and Nitrobacter handle nitrification, while Rhizobium and Azotobacter handle nitrogen fixation.