Root nodules in leguminous plants:

Options

A. Are large outgrowths
B. Are produced due to the activity of Nostoc
C. Establish a direct vascular connection with host
D. Are pink due to nitrogenase

Answer

### Step-by-Step Solution:

1. **Understanding Root Nodules**:
- Root nodules are specialized structures formed on the roots of leguminous plants. They are primarily associated with the symbiotic relationship between the plant and certain bacteria, particularly Rhizobium.

**Hint**: Remember that root nodules are not just random structures; they have a specific biological function and origin.

2. **Formation of Root Nodules**:
- The formation of root nodules occurs when Rhizobium bacteria infect the roots of leguminous plants. This interaction leads to the development of nodules where the bacteria reside.

**Hint**: Think about the relationship between the bacteria and the plant; it’s a mutualistic relationship.

3. **Function of Root Nodules**:
- Root nodules serve as sites for nitrogen fixation. The bacteria within the nodules convert atmospheric nitrogen (N2) into ammonia (NH3), which the plant can use for growth.

**Hint**: Consider why nitrogen fixation is important for plants and how it benefits them.

4. **Vascular Connection**:
- Root nodules establish a direct vascular connection with the host plant. This connection allows for the exchange of nutrients between the plant and the bacteria.

**Hint**: Think about how nutrients are transported in plants and why this connection is crucial.

5. **Presence of Hemoglobin**:
- The nodules contain a red or pink pigment known as leghemoglobin. This pigment helps in the transport of oxygen to the bacteria while preventing the free oxygen from interfering with the nitrogen-fixing process.

**Hint**: Remember that leghemoglobin is similar to hemoglobin in animals; it plays a crucial role in the function of root nodules.

6. **Evaluating Options**:
- Based on the information, we can evaluate the options provided:
- **(A)** Large outgrowth: Incorrect, as root nodules are not simply large outgrowths.
- **(B)** Produced due to the activity of Nostoc: Incorrect, as they are specifically associated with Rhizobium.
- **(C)** Establish a direct vascular connection with the host: Correct, as this is a key function of root nodules.
- **(D)** Pink due to nitrogenous: Incorrect, as the pink color is due to leghemoglobin, not nitrogenous.

**Hint**: When evaluating options, always refer back to the key functions and characteristics of the subject matter.

### Summary:
Root nodules in leguminous plants are formed through the interaction between Rhizobium bacteria and the plant roots. They facilitate nitrogen fixation and establish a vascular connection for nutrient exchange, containing leghemoglobin which gives them a pink color.