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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]

Gravitropism is directional growth of a plant organ in response to gravity. A downward-growing root shows a positive response; an upward-growing shoot shows a negative one. Reorienting a growing plant can therefore produce curvature as growth becomes unequal on opposite sides. The response concerns the direction of growth, rather than passive bending under the organ's weight.

In Arabidopsis thaliana, auxin links perception near the root tip to growth farther back. Ranjan Swarup and collaborators used tissue-targeted expression of the influx carrier AUX1 to map where transport was needed. Their 2005 experiments implicated the lateral root cap and elongating plant epidermis. Separately, blocking auxin responsiveness in expanding plant epidermis cells abolished the gravity response. The comparison distinguishes delivery of a signal from the capacity of receiving cells to respond to it.^[ref]

A horizontal root provides a useful observation: record the tip position over time and inspect curvature through the root elongation zone, rather than expecting the entire existing root to rotate as a rigid rod. Holding the light direction fixed helps distinguish a gravity response from phototropism. Transport, cellular growth and final tip direction are related measurements, but they are not interchangeable.

A phototropin is a blue-light-responsive receptor with kinase activity that regulates responses including phototropism, chloroplast relocation and stomatal opening. Light perception changes intracellular regulation, which in turn alters movement or growth.

Mutants defective in the chloroplast high-light avoidance response identified a phototropin homologue required for that response. Comparing movement under strong and weak light helps separate avoidance from the different response of positioning chloroplasts to capture light.^[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]