= Phototropism
<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.<The Power of Movement in Plants by Charles Darwin>[Chapter IX, localized sensitivity of Phalaris tips and transmitted curvature of the lower region]{ref}
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