Final answer:
Phototropism is the bending of seedlings towards light. Charles Darwin and Peter Boysen-Jensen conducted experiments to study the phenomenon. A chemical signal produced in the plant tip is responsible for the bending, and the hormone indole acetic acid (IAA) causes the phototropic response to light.
Step-by-step explanation:
Phototropism is the bending of seedlings towards light. Charles Darwin and his son Francis first described phototropism as the response of plants to light. They observed that light was perceived by the tip of the plant, but the bending response occurred in a different part of the plant. This led them to conclude that a signal traveled from the tip to the base of the plant.
In 1913, Peter Boysen-Jensen conducted an experiment to show that a chemical signal produced in the plant tip was responsible for the bending at the base. He cut off the tip of a seedling and covered the cut section with gelatin. When illuminated, the seedling bent toward the light. However, when impermeable mica flakes were inserted between the tip and the cut base, the seedling did not bend. This suggested that the chemical signal traveled on the shaded side of the seedling. It was also found that the phototropic response involved faster cell elongation on the shaded side than on the illuminated side.
The hormone responsible for the phototropic response to light is indole acetic acid (IAA). As light passes through a plant stem, it diffracts and activates phototropin. This activation occurs mostly on the illuminated side, causing IAA to accumulate on the shaded side. The elongation of stem cells is influenced by IAA, leading to the bending of the plant towards the light.