Tropism

Driesch defined a tropism in 1908 as a directed movement of a growing part of a plant or hydroid, determined by the direction of a directed agent.¹ He said the theory would be simple if only typical cases existed, such as a branch bending to light or roots bending to the ground.¹ He noted two classes of complication. In the second, a change in the general conditions of life alters the sense of a tropism. Noll called this heterogeneous induction.² Driesch added that such a change can restore form. After a pine is decapitated, a side branch takes on the negative geotropism of the lost main axis.²

Loeb held in 1912 that tropisms and tropism-like reactions pave the way for a rationalistic conception of the psychological reactions of animals.³ He explained why an electric current and light can move the same animal differently. The current changes ion concentrations inside the animal and on its surface, while light acts chemically at the surface.³ In 1916 he used tropisms against the idea that caves shaped their fauna. Cave fishes were negatively heliotropic and positively stereotropic, so they would be forced into a cave, and Eigenmann had shown they were adapted to darkness before entering.⁴ In 1918 he cited Holmes, who lit the water scorpion Ranatra from one side. The legs on that side bent and the opposite legs extended, as in forced movements from a sideways galvanic current.⁵

Gershman and colleagues (2021) describe Loeb as the champion of the view that protozoan behaviour was driven by tropisms. Loeb and his students applied the plant concept to algae and animals, treating many movements as approach or avoidance reactions to an asymmetric force.⁸ Jennings, in 1906, argued that learning was one mechanism of protozoan behaviour. He observed that repeated aversive stimulation of the ciliate Stentor roeseli produced a sequence of distinct avoidance behaviours. Because tropisms were usually taken as unlearned stimulus-response mappings, this was a problem for Loeb's theory.⁹ Both men saw connections between animal behaviour and simpler organisms, but they understood those connections differently.⁸

Calvo and Baluška (2015) describe a light escape tropism in roots. Arabidopsis roots produce hydrogen peroxide within seconds of exposure to light, and illumination stress induces the escape.⁶ They note that Drosophila larvae and C. elegans also show light escape, and that these distant organisms use reactive oxygen species as the signal.⁶ Gagliano and colleagues (2016) reported that pea seedlings learned to grow towards the light on the basis of a neutral fan cue. They state that this learned behaviour prevailed over the innate positive tropism to light, and they call their results the first unequivocal evidence of associative learning in plants.⁷