In plant biology, which of the following is the best example of phototropism, that is, growth response to light?

Introduction / Context:
Although classified under general knowledge and physics in the database, this question actually belongs to basic plant biology. Phototropism is the growth response of a plant toward or away from a light source. Understanding this helps explain why plants tend to orient leaves and stems toward light to maximise photosynthesis. This question asks you to identify which example best illustrates phototropism among the given options.

Given Data / Assumptions:
- Phototropism refers specifically to growth responses triggered by light direction.
- Options describe various plant behaviours: roots following water, stems bending toward light, roots growing downward and Venus flytrap movement.
- We assume textbook definitions of tropisms such as phototropism, geotropism (gravitropism) and hydrotropism.

Concept / Approach:
Phototropism is a directional growth response where the stimulus is light. Positive phototropism is growth towards light, while negative phototropism is growth away from light. Stems and shoots of many plants show positive phototropism by bending toward a window or lamp. Root growth toward water is hydrotropism, and root growth downward is geotropism or gravitropism. A Venus flytrap closing on an insect is a rapid movement, not a slow growth response, and is better described as a nastic movement. Therefore, the correct phototropism example is the plant bending toward the light.

Step-by-Step Solution:
Step 1: Recall that phototropism is growth in response to the direction of light. Step 2: Look at option B. A plant bending toward a light source clearly involves light as the stimulus and directional growth. Step 3: Option A describes roots following water, which is hydrotropism, not phototropism. Step 4: Option C describes roots growing into the soil, which is geotropism or gravitropism. Step 5: Option D describes rapid movement of a Venus flytrap, which is a nastic or stimulus-triggered movement, not a tropic growth. Step 6: Therefore, the example that matches phototropism is the plant bending toward the light.

Verification / Alternative check:
Standard biology textbooks often show photographs or diagrams of seedlings placed near a window, where stems bend toward the incoming sunlight. This is labelled as positive phototropism. Roots, on the other hand, usually grow downward regardless of the direction of light, illustrating gravitropism. Venus flytraps close rapidly in response to touch, which is not growth oriented by light. These standard illustrations clearly confirm that stem bending toward light is the classic example of phototropism.

Why Other Options Are Wrong:
Roots of a plant following water: This is an example of hydrotropism, a response to water, not to light.
Roots growing into the soil: This is geotropism or gravitropism, where growth direction is controlled by gravity, not by light direction.
A Venus flytrap capturing a fly: This is a rapid movement triggered by mechanical stimulation, not a slow, directional growth process caused by light.

Common Pitfalls:
The main confusion arises from mixing up different tropisms. Learners may think any directional movement of a plant is phototropism, even if the stimulus is water or gravity. To avoid this, always ask: “What is the stimulus causing the response?” If it is light, then phototropism is involved. If it is gravity, water, or touch, the corresponding tropism or movement has a different name.

Final Answer:
The best example of phototropism is a plant stem bending toward the light source.