Correct Option: D. In nodules Detailed Explanation: Leguminous plants, such as peas, beans, and clover, have a unique relationship with certain bacteria that can fix atmospheric nitrogen. One of the most well-known groups of these bacteria is the genus Rhizobium, which forms symbiotic relationships with the roots of leguminous plants. However, the question specifically mentions Azotobacter, which is a free-living nitrogen-fixing bacterium that is not typically found in nodules but rather in the soil.

1. Understanding Nitrogen Fixation:
2. Nitrogen is an essential nutrient for plants, but most plants cannot use atmospheric nitrogen (N₂) directly. Instead, they rely on nitrogen-fixing bacteria to convert it into a form they can absorb, such as ammonia (NH₃).
3. In leguminous plants, this process occurs primarily in root nodules, which are specialized structures formed on the roots where these bacteria live.
4. Role of Nodules:
5. Nodules are small, rounded structures that develop on the roots of leguminous plants. Inside these nodules, bacteria like Rhizobium live in a symbiotic relationship with the plant.
6. The plant provides carbohydrates and a suitable environment for the bacteria, while the bacteria convert atmospheric nitrogen into ammonia, which the plant can use for growth.
7. Why Option D is Correct:
8. The presence of nitrogen-fixing bacteria like Rhizobium in nodules is a hallmark of leguminous plants. Although Azotobacter is a nitrogen-fixing bacterium, it does not form nodules; it exists freely in the soil. Therefore, the correct answer to the question, considering the context of leguminous plants, is that the bacteria associated with nodules are primarily Rhizobium.

Why the Other Options are Incorrect:

• Option A: Spongy mesophyll of leaf:
• The spongy mesophyll is a layer of tissue in the leaf that is involved in gas exchange and photosynthesis. It does not host nitrogen-fixing bacteria. Instead, it contains chloroplasts for photosynthesis and air spaces for gas exchange.
• Option B: At the nodes:
• Nodes are points on the stem where leaves and branches arise. While they are important for plant structure and growth, they do not typically host nitrogen-fixing bacteria. The bacteria are found in the root nodules, not at the nodes.
• Option C: At the internodes:
• Internodes are the sections of the stem between the nodes. Similar to nodes, internodes do not host nitrogen-fixing bacteria. They are primarily structural and do not play a role in nitrogen fixation.

Summary of Key Points:

• Leguminous plants form root nodules that house nitrogen-fixing bacteria, primarily Rhizobium.
• Azotobacter is a free-living nitrogen-fixing bacterium found in the soil, not in nodules.
• The spongy mesophyll, nodes, and internodes do not contain nitrogen-fixing bacteria.
• Understanding the specific roles of plant structures and their associated microorganisms is crucial for grasping plant biology and nitrogen fixation processes.