Phototropism is directional growth of a plant organ in response to the direction of light. Positive phototropism bends an organ toward light; negative phototropism bends it away. Unlike phototaxis, it changes the growth of an attached organ rather than requiring locomotion of the whole organism.
Charles Darwin, assisted by Francis Darwin, investigated where grass seedlings perceived lateral light. Opaque coverings over the tips suppressed the normal bending response, whereas protecting the lower region while exposing the tip allowed bending below. These comparisons supported transmission of an influence from the illuminated tip to the responding region. They did not identify auxin chemically: the experiment separated the location of perception from the location of growth.[ref]
The Power of Movement in Plants by Charles Darwin. Chapter IX, localized sensitivity of Phalaris tips and transmitted curvature of the lower region
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Phototropism is the growth response of plants to light direction. It is a form of directional growth where plant parts, such as stems and leaves, grow towards a light source. This phenomenon is primarily driven by the plant hormone auxin, which regulates cell elongation. When light is more intense on one side of the plant, auxin accumulates on the shaded side, promoting cell elongation on that side. Consequently, the plant bends toward the light to maximize light absorption for photosynthesis.