On the Role of Auxin in Phototropism and Light-Growthreactions of Avena-Coleoptiles

In 1909 Blaauw, discovering — simultaneously with Froschel (1908) — the stimulus quantity law, tried to explain phototropism as well as changes in phototonus in terms of photochemical reactions. Negative phototropism of normally positive phototropic organs, occurring after the exposure to high amounts of light energy, were compared to the solarization of the photographic plate. Blaauw thus advocated a unity of the mechanism of both the positive and the negative reaction. A few years later (1914,1915,1918) Blaauw stated that all-round illumination causes light-growthreactions, their magnitude largely depending on the light quantities. Since at unilateral illumination both halves of the phototropic organ are irradiated with different amounts of light energy, this illumination would cause an unequal change in the growth rate of the light-(L) and of the dark-(D) side of the organ. Phototropism after unilateral illumination therefore would represent a special case of the light-growthreactions after all-round illumination, the latter being the primary phenomenon {1915, page 187): ,,Die Lichtwachstumsreaktion ist die primate, der Phototropismus die secundare Erscheinung, welche notwendig aus ihr erfolgt, wenn durch ortlich ungleiche Belichtung ortlich ungleiche Wachstumsreaktionen entstehen.”