A mechanism to prevent cross pollination is

Options

A. Protogyny
B. Protandry
C. Heterostyly
D. Cleistogamy.

Answer

To solve the question "A mechanism to prevent cross pollination is," we will analyze the given options step by step.

### Step 1: Understand Cross Pollination
Cross pollination occurs when pollen from the anther of one flower fertilizes the stigma of another flower. To prevent this, certain mechanisms can be employed by plants.

**Hint:** Remember that cross pollination involves two different flowers, so mechanisms that promote self-pollination or keep flowers closed are relevant.

### Step 2: Analyze the Options
1. **Protogyny**: In protogyny, the female reproductive organs (gynosium) mature before the male reproductive organs (androsium). This mechanism prevents self-fertilization but does not necessarily prevent cross pollination.

**Hint:** Think about whether this mechanism allows for pollen transfer from other flowers.

2. **Protoendry**: In protoendry, the male reproductive organs mature before the female reproductive organs. Similar to protogyny, this prevents self-fertilization but does not prevent cross pollination.

**Hint:** Consider if this mechanism allows for the possibility of cross pollination.

3. **Heterostyle**: Heterostyly involves flowers having different styles, which can help in promoting cross pollination rather than preventing it. Thus, it does not serve to prevent cross pollination.

**Hint:** Reflect on how flower structure influences pollination types.

4. **Clistogamy**: Clistogamous flowers remain closed and do not open. In this case, the anthers and stigma are close together, leading to self-pollination. This mechanism effectively prevents cross pollination since the flowers do not open to allow pollen from other flowers.

**Hint:** Think about whether the flower's structure allows for interaction with other flowers.

### Step 3: Conclusion
After analyzing all the options, the correct answer to the question "A mechanism to prevent cross pollination is" is **Clistogamy**.

**Final Answer:** Clistogamy