The correct option is D. Positively hydrotropic. Explanation of the Correct Answer

1. Understanding Hydrotropism: Hydrotropism is the growth response of a plant to moisture gradients in the soil. Roots exhibit positive hydrotropism, meaning they grow towards areas of higher moisture content. This is crucial for the plant's survival, as it allows the roots to access water, which is essential for various physiological processes, including photosynthesis, nutrient transport, and maintaining turgor pressure.
2. Mechanism of Hydrotropism: When roots sense moisture, they produce growth hormones, particularly auxins. These hormones promote cell elongation on the side of the root that is away from the moisture, causing the root to bend towards the water source. This growth response ensures that the plant can efficiently absorb water from the soil.
3. Importance of Water for Plants: Water is vital for plants as it serves multiple functions:
4. It is a solvent for nutrients, allowing them to be transported within the plant.
5. It helps maintain cell turgor, which is necessary for structural integrity.
6. It is involved in photosynthesis, where it is split to release oxygen and provide electrons for the synthesis of glucose.

Why the Other Options are Incorrect

• A. Positively phototropic: This option refers to the growth of plants towards light, which is known as phototropism. While roots do exhibit some degree of phototropism, they are primarily negatively phototropic, meaning they grow away from light. This is because roots are typically found underground, where light is not available, and their primary function is to anchor the plant and absorb water and nutrients.
• B. Negatively geotropic: Geotropism refers to a plant's growth response to gravity. Roots are positively geotropic, meaning they grow downwards towards the gravitational pull. This is essential for anchoring the plant and ensuring that roots can access water and nutrients from the soil. Therefore, this option is incorrect.
• C. Negatively hydrotropic: This option suggests that roots grow away from moisture, which is the opposite of what actually occurs. Roots are positively hydrotropic, meaning they grow towards moisture. This response is critical for the plant's ability to survive in varying soil moisture conditions. Thus, this option is also incorrect.

Summary of Key Points

• Roots of plants exhibit positive hydrotropism, growing towards areas of higher moisture.
• This growth response is mediated by auxins, which promote cell elongation on the side of the root away from moisture.
• Water is essential for plant survival, aiding in nutrient transport, maintaining turgor, and participating in photosynthesis.
• Roots are negatively phototropic and positively geotropic, growing away from light and downwards due to gravity, respectively.

By understanding these concepts, students can appreciate the adaptive strategies plants use to thrive in their environments.