Considerations of geotropism in plants

Four periods of research in the reaction of plants to gravity are distinguished: (1) An early period of purely mechanistic interpretation of the phenomena observed. This period, starting in 1700, is characterized by studies of French and British plant physiologists. They demonstrated that gravity influenced the growth direction of plant parts. (2) In 1868 Frank introduced the term geotropism and postulated an active, peculiar force within the plant which causes a physiological reaction of the plant organ. This is the beginning of the second period, directed by consideration of the physiology of irritability. The elucidation of the stimulus-reaction chain, a quantitative description of the phenomena observed, including terminological questions, and the search for geosensors (statolith starch) dominate this "German" period. (3) With the detection of plant hormones to regulate cell elongation in plants at the beginning of the 20th century, the third period begins with the lateral-auxin-distribution theory of Cholodny and Went (1926). (4) After a "silent" period of 25 years research in geotropism of plants was revived with modern, critical studies using refined methods. It was found that there might be different principles of geo-reactions in unicellular geotropic plant organs, in roots, and shoots, respectively. In shoots the asymmetric distribution of auxin might be caused by differences in the longitudinal auxin transport, possibly as a result of an asymmetry of active Golgi vesicles. This hypothesis is discussed in connection with some recent papers on this subject.