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]
The Power of Movement in Plants by Charles Darwin. Chapter IX, Phalaris canariensis, pp. 469–474: glass tubes, opaque caps and transparent controls
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