Gravitation and phase displacement as the basis of geotropism

Darwin treated geotropism as one of several agencies that modify circumnutation, which he regarded as always in progress.¹ Darwin did not propose a cellular mechanism here. Washburn's 1908 textbook reviewed explanations for gravity responses in animals. One held that negatively geotropic animals swim toward greater resistance. Washburn, following earlier critics, said this assumes the fact to be explained. Washburn also cited experiments showing that animals in fluids of their own density, with no weight, still display negative geotropism.⁷

He recalled that eight years earlier he had suggested that cells of geotropic organs hold nonmiscible substances. Changing the organ's position would shift the relative position of these phases.³ He was led to this by muscle. A frog muscle stretched by a weight produced more lactic acid than an unstretched one, an effect he attributed to a change in the form of the muscle.⁴

Bose approached geotropism through excitation. He argued that an apogeotropic shoot responds to gravity as to any other stimulus. A root placed horizontally bends toward gravity, which seemed to differ, but he held that roots respond like all other organs.⁵ To avoid mechanical effects that mask the response, he recorded an electrical signal. He reported a geo-electric response within one minute of applying the gravitational stimulus.⁶ In 1923 he added that rising temperature diminishes the geotropic response and falling temperature, within limits, enhances it.⁸

A 2017 paper on slime mould summarised later findings on Physarum. Its authors wrote that the organism is gravisensitive and positively geotropic. Experiments with respiratory inhibitors in a clinostat suggested the receptor site may be correlated with the mitochondria, though no mechanism was given.⁹ This differs from Loeb's phase-shift picture, which located the effect in the position of non-mixing phases within cells.