Willem Hendrik Arisz became a member of the Royal Botanical Society of the Netherlands in 1909. In 1912 he passed his “doctoraal” examination with honours. In those days his interest was directed mainly towards animal physiology. However, after a short stay at the Laboratory for Marine Biology in Naples and at the Botanical Laboratory of the University of Bonn, he chose a botanical subject, phototropism, for his doctorate, which he took in 1914, this also with honours. His professor and promoter was Prof. Dr. F. A. F. C. Went. Research on the stimulation of plants in those days was strongly influenced by animal physiology. Arisz investigated a series of phenomena known as “tonus phenomena”. His work was influenced by that of A. H. Blaauw who, in 1909, had found that a certain quantity of radiation energy is necessary for the occurrence of a perceivable curvature (irradiation time X intensity).