The history of discovery of auxin is a fascinating chapter in plant physiology. The hormone auxin was discovered first, through some elegant experiments by Charles Darwin and his son Francis around 1880. They found that if a coleoptile of a grass (Phalaris) was illuminated from one side it bends towards light. However, if the tip region was covered, no bending occurred. In his book "The Power of Movement in Plants" published in 1880, Darwin wrote that some matter in the upper part, which is acted upon by light, transmits its effect to the lower part causing the latter to bend.
Figure: Darwin's experiment on canary grass coleoptiles