The discovery of the chemical nature of the plant hormone auxin.

The concept of substances working as a chemical messenger among the plant tissues was guessed in the last quarter of the nineteenth century as a consequence of a series of observations and experiments concerning two important phenomena: the geotropism and the heliotropism. The work of Theophil Ciesielski, Charles and Francis Darwin, Julius von Sachs, Martinus Beijerinck and Julius Wiesner supplied the fundamental pillar to the modern plant physiology. Hans Fitting [1909] introduced the term "hormone", coined in 1902 to indicate a substance promoting chemical correlations among various organs of animals, in plant physiology for indicating a substance stimulating the development of the ovary of orchid flower. Paul Boysen-Jensen and Arpad Paál focused the occurrence of a growth substance that somehow regulated the positive curvature of oats coleoptiles, the distinctive feature of the phototropism. During the 1920s, a few Mitteleuropean botanists gave circumstantial evidence of such a substance before the Dutch physiologist Frits Went elaborated an experimental procedure for isolating it, and quantifying its physiological activity. Went's work crowned with success a half century of research and opened the door to the chemistry of the auxins. A next important step concerned the purification of sufficient amounts of substance for analytical purposes. Five years of attempts made by Hermann Dolk, Jan Haagen-Smit, F. Kögl and Kenneth Thimann had success and the "substance" was finally identified as indolacetic acid and named "auxin". This result delivered definitively the concept of plant growth from a secular mysticism and established a milestone in the modern plant physiology.