It is a common observation that roots of plants always grow toward the source of water. The given phenomenon is explained by the concept of

Options

A. gravitropism
B. hydrotropism
C. phototropism
D. thermotropism

Answer

To solve the question about why roots of plants grow toward the source of water, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Observation**: The question states that roots of plants grow toward the source of water. This is a common observation in botany.

2. **Identifying the Options**: The options provided are:
- Gravitropism
- Hydrotropism
- Phototropism
- Thermotropism

3. **Defining the Terms**:
- **Gravitropism**: This refers to the growth of plants in response to gravity. Roots typically grow downward due to this phenomenon.
- **Hydrotropism**: This is the growth of plant roots towards moisture or water. The prefix "hydro" refers to water.
- **Phototropism**: This is the growth of plants in response to light, where stems often grow towards light sources.
- **Thermotropism**: This refers to the growth of plants in response to temperature changes.

4. **Analyzing the Relevant Phenomenon**: Since the roots are specifically growing towards water, we need to focus on the phenomenon that involves water as a stimulus.

5. **Conclusion**: The correct answer is **Hydrotropism**, as it specifically describes the growth of roots in response to the presence of water.

### Final Answer:
The phenomenon explained by the roots of plants growing toward the source of water is **Hydrotropism**.

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