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Francis Darwin (1848–1925) was a botanist who collaborated with Charles Darwin on experimental plant physiology. The title page of The Power of Movement in Plants by Charles Darwin, published in 1880, explicitly credits his assistance.

Their phototropism experiments compared seedlings with transparent versus opaque coverings over their tips. Blocking light at the tip greatly reduced bending below it, whereas transparent coverings allowed bending. The comparison separated the location of light perception from the region that curved, suggesting transmission of an influence down the growing structure. It did not identify the chemical nature of that influence.^[ref]

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]