Pulvinus is
(a) Swollen leaf base
(b) Cylindrical stalk that joins the leaf base with lamina
(c) Lateral small leaf like structure
(d) Modified petiole

Options

A. Swollen leaf base
B. Cylindrical stalk that joins the leaf base with lamina
C. Lateral small leaf like structure
D. Modified petiole

Answer

### Step-by-Step Solution:

1. **Understanding the Term "Pulvinus":**
- The term "pulvinus" refers to a specific structure found at the base of certain leaves. It is important to identify its characteristics and functions.

2. **Analyzing the Options:**
- (a) Swollen leaf base: This option describes a pulvinus accurately.
- (b) Cylindrical stalk that joins the leaf base with lamina: This does not describe a pulvinus.
- (c) Lateral small leaf-like structure: This is not the definition of a pulvinus.
- (d) Modified petiole: While related to leaf structure, this does not specifically define a pulvinus.

3. **Identifying the Correct Answer:**
- Based on the definitions provided, the correct answer is (a) Swollen leaf base. This is because a pulvinus is indeed a swollen structure at the base of the leaf.

4. **Understanding the Function of Pulvinus:**
- The pulvinus is commonly found in leguminous plants and is responsible for the pulvinar movement, which helps in the movement of leaves, particularly in response to light and darkness. This movement protects the young axillary bud.

5. **Conclusion:**
- Therefore, the correct answer to the question "Pulvinus is" is (a) Swollen leaf base.