Which of the following statements is not correct?

Options

A. Pollen grains of many species can germinate on the stigma of a flower, but only one pollen tube of the same species grows into the style
B. Insects that consume pollen or nectar without bringing about pollination are called pollen/ nectar-robbers
C. pollen germination and pollen tube growth are regulated by chemical components of pollen interacting with those of the pistil
D. Some reptiles have also been reported as pollinators in some plant species

Answer

To determine which statement is not correct regarding sexual reproduction in flowering plants (angiosperms), we will analyze each statement provided in the question.

### Step-by-Step Solution:

1. **Identify the Statements**: We need to read and understand each statement related to the process of sexual reproduction in flowering plants.

2. **Evaluate Statement A**:
- Statement A claims that many species of pollen grains can germinate on the stigma of a flower.
- This statement is incorrect because pollen grains can only germinate on the stigma if they are compatible. A pollen grain from a different species will not germinate on the stigma of another species.

3. **Evaluate Statement B**:
- Statement B states that insects that consume pollen and nectar without pollination can be called pollen or nectar robbers.
- This statement is correct. Insects that take these resources without aiding in pollination are indeed referred to as robbers.

4. **Evaluate Statement C**:
- Statement C mentions that when chemical components of pollen grains interact with chemical components of the pistil, pollen germination and pollen tube growth occur.
- This statement is also correct. The interaction between compatible pollen and the stigma is essential for germination and subsequent pollen tube growth.

5. **Evaluate Statement D**:
- Statement D indicates that in some reptiles and plant species, pollinators have been mentioned.
- This statement is correct as well. While insects are the most common pollinators, some reptiles and other animals can also assist in pollination.

6. **Conclusion**: After evaluating all the statements, we find that Statement A is the only incorrect statement.

### Final Answer:
The statement that is not correct is:
**A. Many species of pollen grains can germinate on the stigma of a flower.**