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Tropism

A tropism is a directed movement of a growing part of a plant or hydroid, set by the direction of an acting agent such as light or gravity.2 Loeb extended the idea to whole animals and argued that it could support a rationalistic account of animal behaviour.4

Earliest held
1880, Darwin, C.
Most discussed in
Plant Response as a Means of Physiological…, 1906
In the library
483 passages in 15 works
Rewritten
2026-10-03
18501875190019251950197520002025Darwin, C., 18801Driesch, H., 19082Driesch, H., 19083Loeb, J., 19124Loeb, J., 19125Loeb, J., 19166Loeb, J., 19167Child, C. M., 19248Child, C. M., 19249
01

Plant movements and definition

Darwin treated heliotropism, apogeotropism and geotropism in The Power of Movement in Plants (1880) as movements related to circumnutation. He suggested that a species which had ceased to be heliotropic might become apheliotropic through the preservation of individuals that circumnutated away from the light, and he admitted that the cause of such variation was unknown.1 Driesch (1908) defined a tropism as a directed movement of a growing part of a plant or hydroid, determined by the direction of a directed agent.2 He added that the theory would be simple if only typical cases existed, and that complications arise when conditions change.3 One example is decapitation of a pine, after which a side branch takes over the negative geotropism of the lost axis.3

02

Loeb's physical programme

Loeb (1912) held that tropisms and tropism-like reactions pave the way for a rationalistic conception of the psychological reactions of animals.4 He explained why an animal may respond differently to an electric current than to light. The current changes ion concentrations inside the tissue and on its surface, whereas the chemical action of light is limited to the surface.5 In 1916 he reported Eigenmann's finding that cave animals were already negatively heliotropic and positively stereotropic. These tropisms would force them into a cave, so the cave did not adapt them to darkness.6 Loeb also proposed the term segmental tropisms for cases where one body segment overpowers the rest, in place of the term reflex.7

03

Loeb against Jennings

Child (1924) noted that the term tropism had been used loosely. In Loeb's sense, he wrote, a tropism postulates an effective integration pattern that gives the reaction directly.8 Jennings took a different view. For him such a pattern need not be predetermined or inherited, and it may arise in the individual from successful trial behaviour.9 Child accepted part of each position. He held that some tropism-like patterns belong to the normal pattern of the species and relate to the axial gradients, while others are built up on a trial foundation. On either view, he said, a tropism needs a behaviour mechanism already adjusted to the case.9

SourcesEach quotation was checked word for word against the passage it opens.
  1. A species which has ceased to be helio- tropic might also be rendered apheliotropic by the preservation of the individuals which tended to cir- cumnutateDarwin, C., 1880 · The Power of Movement in Plants · open at passage 968
  2. is a directed movement of a growing part of a plant or hydroid determined by the direction of a directed agent.Driesch, H., 1908 · The Science and Philosophy of the Organism · open at passage 27
  3. if a pine is decapitated, one of the side branches assumes the negative geotropism of the lost main axis, and a similar phenomenon holds for roots.Driesch, H., 1908 · The Science and Philosophy of the Organism · open at passage 30
  4. Tropisms and tropism-like reactions are elements which pave the way for a rationalistic conception of the psychological reactions of animalsLoeb, J., 1912 · The Mechanistic Conception of Life · open at passage 110
  5. the electric current causes changes in the concentration of ions within, as well as upon the surface, while the chemical action of light is essentially limited to the surface.Loeb, J., 1912 · The Mechanistic Conception of Life · open at passage 110
  6. These animals are all negatively heliotropic and positively stereotropic, and with these tropisms they would be forced to enter a cave whenever they are put at the entrance.Loeb, J., 1916 · The Organism as a Whole, from a Physicochemical Viewpoint · open at passage 493
  7. it is better to call them tropisms since the organism as a whole is involved. If necessary we might speak of segmental tropisms.Loeb, J., 1916 · The Organism as a Whole, from a Physicochemical Viewpoint · open at passage 444
  8. The tropism in Loeb's sense (pp. 230-231) postulates the presence of an effective integration patternChild, C. M., 1924 · Physiological Foundations of Behavior · open at passage 684
  9. such a pattern is not necessarily predetermined or inherited, but may have developed in the individual as the result of suecessful trial behaviorChild, C. M., 1924 · Physiological Foundations of Behavior · open at passage 684
Linked ideas
Trial and errorin tension with
Child's note pairs the tropism theory (Loeb) with the trial-and-error theory (Jennings) as rival accounts of animal behaviour.
Galvanotaxisrelated to
Loeb says an electric current moves an animal differently from light because it changes ion concentrations inside and on the surface.
Regenerationrelated to
Driesch says a changed tropism can act restitutively: a side branch takes on the negative geotropism of a decapitated pine's lost axis.
Loeb offers tropisms as elements for a mechanistic account of animal behaviour, the same ground Jennings's trial-and-error work covers.
Polarityrelated to
Driesch's negative geotropism of the main axis, taken over by a side branch, ties a directed response to the organism's axis.
Criteria of mindin tension with
Loeb read animal responses as unconscious tropisms like those of plants, against readings that infer sensation or mind from the same behaviour.
Verworn and Jennings describe crowds of swarm-spores gathering as individual light responses, not as a group-level intelligence.
Tropisms are the relatively fixed reaction types set by usual conditions of existence, yet Jennings says even these are not completely fixed or invariable.
Circumnutationprecursor of / follows
Darwin, as reported by Bose, held that heliotropic movements are circumnutation modified in direction; Bose notes the crucial test could not be applied.
Darwin and Bose both discuss movements excited by light and gravitation, and Bose describes tip stimulation producing curvature.
Heliotropism and geotropism are the standard cases in Driesch's definition of a tropism as a directed movement set by a directed agent.
Whiting describes growth toward food and away from light as innate behaviour that can be read as computation, so tropism and decision may overlap.
Child linked inherited tropism-like reaction patterns to the axial gradients of the organism.
Loeb's tropism theory was a mechanistic route to behaviour; Driesch wrote on tropisms from his own standpoint.
Passages tie tropism to differences between two sides: Jennings/Davenport's lateral illumination of a bilateral worm, Bose's torsion from differential excitability of two halves. None describes body left-right asymmetry itself.
Jennings describes green organisms turning back from darkness, a response that follows from their oxygen supply.
Where it is discussedPassages matching tropism, tropisms, heliotropism, forced movements
1906Plant Response as a Means of Physiological Investigation · Bose, J. C.152
1880The Power of Movement in Plants · Darwin, C.113
1912The Mechanistic Conception of Life · Loeb, J.63
1916The Organism as a Whole, from a Physicochemical Viewpoint · Loeb, J.39
1906Behavior of the Lower Organisms · Jennings, H. S.34
1926The Nervous Mechanism of Plants · Bose, J. C.20
1924Physiological Foundations of Behavior · Child, C. M.17
1908The Animal Mind: A Textbook of Comparative Psychology · Washburn, M. F.13
1908The Science and Philosophy of the Organism · Driesch, H.12
1926A Bipolar Theory of Living Processes · Crile, G. W.6