The hormone responsible for phototropism is

Options

A. Auxin
B. `C_2 H_4`
C. ABA
D. GA

Answer

To answer the question "The hormone responsible for phototropism is," we can follow these steps:

### Step 1: Understand Phototropism
Phototropism is the growth response of a plant to light. The term "photo" refers to light, and "tropism" refers to movement or growth. In phototropism, plants grow towards the light source.

**Hint:** Remember that phototropism involves plant growth in response to light direction.

### Step 2: Identify the Hormones Involved
There are several plant hormones, but the key hormones to consider in the context of phototropism are auxins, ethylene, abscisic acid, and gibberellins.

**Hint:** Focus on the role of auxins in plant growth and movement.

### Step 3: Recall Key Experiments
Charles Darwin and his son, Francis Darwin, conducted experiments on canary grasses to study phototropism. They found that when light was directed from one side, the plant would bend towards the light. This bending is due to the distribution of auxins.

**Hint:** Think about the experiments that demonstrated how plants respond to light.

### Step 4: Determine the Role of Auxins
Auxins are the hormones responsible for the elongation of cells on the side of the plant that is away from the light. This unequal growth causes the plant to bend towards the light source.

**Hint:** Auxins are crucial for the bending movement in phototropism.

### Step 5: Conclude with the Correct Answer
Based on the understanding of phototropism and the role of auxins, we conclude that the hormone responsible for phototropism is **auxin**.

**Final Answer:** The hormone responsible for phototropism is auxin.