The continuous upward flow of water stream in tall trees is maintained due to ___________

Options

A. Guttation and transpiration
B. Transpiration pull only
C. Cohesive force between water molecules only
D. Cohesive force between water molecules and transpiration pull

Answer

To solve the question, "The continuous upward flow of water stream in tall trees is maintained due to ___________", we need to analyze the options provided and understand the mechanisms involved in the ascent of sap in plants.

### Step-by-Step Solution:

1. **Understanding Water Movement in Plants**:
- Water from the soil enters the plant through root hairs and moves up through the xylem tissues. This process is known as the ascent of sap.

2. **Role of Transpiration**:
- Transpiration is the process where water evaporates from the leaves of the plant. This loss of water creates a negative pressure or tension in the xylem, which helps pull water upward from the roots. This phenomenon is known as the transpiration pull.

3. **Cohesive Forces**:
- Water molecules exhibit cohesive forces, meaning they are attracted to each other. This cohesion helps maintain a continuous column of water in the xylem, preventing the water column from breaking as it moves upward.

4. **Combining the Concepts**:
- The continuous upward flow of water in tall trees is not solely due to transpiration or cohesive forces alone. Both transpiration pull and the cohesive forces between water molecules work together to ensure that water can move efficiently from the roots to the leaves.

5. **Evaluating the Options**:
- The options given were:
- Guttation and transpiration
- Transpiration pull only
- Cohesive force between water molecules only
- Cohesive force between water molecules and transpiration pull
- Based on our understanding, the correct answer is the last option: "Cohesive force between water molecules and transpiration pull."

### Final Answer:
The continuous upward flow of water stream in tall trees is maintained due to **cohesive force between water molecules and transpiration pull**.