Heliotropism, geotropism and related tropic responses in plants

Darwin's 1880 book treated heliotropism and apheliotropism as modified forms of circumnutation. Its contents list the steps by which one movement is converted into the other.² He argued that a species which had ceased to be heliotropic might become apheliotropic through the preservation of individuals whose circumnutation ran against the direction of the light. He added that the cause of this and most other variations is unknown.¹ The same contents list diaheliotropism, which Darwin also called transversal heliotropism. It is described there as influenced by epinasty, the weight of the part and apogeotropism.² The chapter on geotropism covers the movements of radicles and the burying of seed-capsules.¹

Bose, in 1906, argued against assuming a separate dia-geotropic property in leaves. He said the observed effects follow from differential geotropic sensibility in a dorsiventral organ, and he took the same view of dia-heliotropism.³ Driesch, in 1908, wrote that a theory of tropisms would be simple if only typical cases existed, such as a branch bending to light or roots bending toward the ground.⁴ His second class of complications arises when the general conditions of life change. A change in the temperature of the medium can change the sense of heliotropism, which Noll called heterogeneous induction.⁵ Driesch noted that after a pine is decapitated, a side branch takes on the negative geotropism of the lost main axis.⁵

Loeb, in 1912, reported experiments showing that the heliotropic reactions of animals are identical with those of plants.⁷ He held that it is erroneous to explain animal heliotropism by features such as vision, which plants lack. His 1916 book reported that a trace of carbon dioxide made negatively heliotropic or indifferent Daphnia, copepods and Volvox violently positively heliotropic.⁸ Washburn, in 1908, described copepods whose normal geotropism was positive but became negative in darkness. She said the light acted by producing a physiological change, not as a directive stimulus.⁶

Evangelidis and colleagues, in 2017, described the slime mould Physarum polycephalum as gravisensitive and positively geotropic. They cited earlier work for this and relied on it to explain why plasmodium navigates around and through elevations on a 3D terrain. The earlier data showed gravisensitivity without proposing a mechanism. Inhibitors of respiration, used in a clinostat, suggested that the receptor site may be correlated with the mitochondria.⁹ The same paper cites a report that the ectoplasmic wall of a tube is thinner on the side nearer the earth. It also cites a report that final sorocarp height depends partly on the gravity environment.⁹