Final answer:
Phototropism occurs as the plant's growth hormone auxin redistributes in response to light, causing the plant to bend towards the light for better photosynthesis. This was first studied by Charles Darwin, who found that the response begins at the apical meristem. Agar blocks were used to identify that a substance, auxin, was responsible for this growth response.
Step-by-step explanation:
Explanation of Phototropism in Plants
Phototropism is the directional growth of a plant in response to light, and it happens because plants are trying to maximize their exposure to light for photosynthesis. Charles Darwin and his son Francis were among the first to study phototropism, discovering that the signal for this movement originates in the growing tip of the plant, specifically the apical meristem. They observed that though the tip of the plant detects the light, the bending occurs at a different part, indicating that the signal must travel from the tip to other parts of the plant for the response to occur.
An important discovery was made using an agar block, which helped scientists demonstrate that a substance was being transferred through the plant to cause the phototropic response. This substance was later identified as the plant hormone auxin, which accumulates on the darker side of the plant, causing cells to elongate and the plant to bend towards the light. Phototropism has been extensively studied, and it is understood that phototropins, a group of blue-light receptors, play a key role in this process alongside auxins.