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Classic experiments on tropic responses. (A) Gravitropism: A potted plant realigns itself with gravity (13). (B) Thigmotropism: A twining vine develops curvature when in contact with a pole (6). (C) Phototropism: A plant reorients itself toward the light source [18th century experiments by Bonnet (14), correctly interpreted by Duhamel du Monceau (15, 16)].
