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Heliotropism as symmetry of photochemical effect (Loeb)

Loeb explained heliotropic bending as a matter of symmetry. Light striking one side of an organ acts differently there than on the other side. The organ turns until symmetrical points receive equal light, and then it stops turning.1 He used the hydroid Eudendrium to argue that animals and plants respond to light by the same rules.5

Earliest held
1880, Darwin, C.
Most discussed in
Plant Response as a Means of Physiological…, 1906
In the library
241 passages in 14 works
Rewritten
2026-11-05
18501875190019251950197520002025Loeb, J., 19001Loeb, J., 19062Loeb, J., 19063Bose, J. C., 19064Loeb, J., 19125Loeb, J., 19166Loeb, J., 19187Loeb, J., 19188Bose, J. C., 19269
01

The symmetry argument

In 1900 Loeb described a stem of the hydroid Eudendrium raised near a window. It bent toward the window like a positively heliotropic plant. Light struck the stem from the side. The protoplasm on that side contracted and resisted lengthening more than the far side did.1 The stem therefore became concave toward the light. Once it pointed along the rays, all symmetrical elements were struck at the same angle. Light intensity was equal at symmetrical points, and nothing remained to turn the stem.1 Later he stated that the heliotropic reactions of animals are identical with those of plants.5

02

Contraction rather than growth

In 1906 Loeb gave the mechanism more detail. He wrote that the region behind the polyp has contractility. The stem shortens more on the more strongly lit side than on the opposite side.2 He added that the curvature in Eudendrium is a phenomenon of contractility and not of growth, although growth may accidentally occur at the same time.3 Bose, writing in the same year, worked mainly on plants. He recorded that Sachs saw negative heliotropic curvature of Tropceolum majus only in summer. Bose attributed this to transmitted excitation, which is helped by higher conductivity of the tissue.4

03

Light dose and wavelength

Loeb treated light as acting chemically. The 1916 text states that the photochemical effect equals intensity times duration, the law of Bunsen and Roscoe. It was tested on regenerating Eudendrium polyps by timing how long it took for half of them to bend. Intensity was varied by changing the distance of the source.6 The 1918 text reports that Ewald and Loeb tested this law on Eudendrium.7 The same text reports that Loeb and Wasteneys found the most efficient part of the spectrum for Eudendrium to lie in the blue, as in some plants.8

04

Bose's alternative reading

In 1926 Bose gave an account of heliotropic curvature in growing organs. A growing organ lit from one side is stimulated directly on the near side and indirectly on the far side. Growth is retarded on the near side and accelerated on the far side, which produces positive curvature.9 He held that under intense or long stimulation the excitation reaches the far side. This neutralises the positive curvature or reverses it to negative.9 Loeb located the Eudendrium effect in contractility instead.3

SourcesEach quotation was checked word for word against the passage it opens.
  1. all the symmetrical elements are struck by the light at the same angle. The intensity of light is thus equal at symmetrical points, and there is no longer occasion for the stem to leave this direction.Loeb, J., 1900 · Comparative Physiology of the Brain and Comparative Psychology · open at passage 268
  2. The heliotropic curvature consists here in the stem undergoing a stronger contraction or short- ening on the more strongly illuminated side of the polyp than on the opposite side.Loeb, J., 1906 · The Dynamics of Living Matter · open at passage 327
  3. The heliotropic curvature in Eudendrium is therefore a phenomenon of contractility and not a phenomenon of growth, although growth may accidentally occur at the same time.Loeb, J., 1906 · The Dynamics of Living Matter · open at passage 328
  4. it is interesting to note that the negative heliotropic curvature of Tropceolum majus, due to transmitted excitation, has only been observed by Sachs during summerBose, J. C., 1906 · Plant Response as a Means of Physiological Investigation · open at passage 1444
  5. In a series of experiments I have shown that the heliotropic reactions of animals are identical with the heliotropic reactions of plants.Loeb, J., 1912 · The Mechanistic Conception of Life · open at passage 53
  6. The experiments were carried out on young regenerating polyps of Eudendrium by measuring the time required to cause fifty per cent, of the polyps to bend to the source of light.Loeb, J., 1916 · The Organism as a Whole, from a Physicochemical Viewpoint · open at passage 413
  7. Ewald and the writer tested the validity of the law of Bunsen and Eoscoe for the heliotropic curvatures of Eudendrium.Loeb, J., 1918 · Forced Movements, Tropisms, and Animal Conduct · open at passage 139
  8. They found, by a method similar to that used by Blaauw, that for the heliotropic curvature of the animal Eudendrium the most efficient part of the spectrum lies in the blueLoeb, J., 1918 · Forced Movements, Tropisms, and Animal Conduct · open at passage 183
  9. Under intense or long continued stimulation, the excitation reaches the distal side, neutralising the positive curvature, or reversing it to negative.Bose, J. C., 1926 · The Nervous Mechanism of Plants · open at passage 216
Linked ideas
The wider treatment of orientation to light and gravity, of which the Eudendrium symmetry argument is one case.
Tropismpart of / contains
Loeb's heliotropism is his model case of a tropism as a forced, symmetry-governed orientation.
Loeb's 1918 book on forced movements contains the Bunsen-Roscoe tests and the wavelength results.
The intensity-times-time law links dose of light to the size of the response.
Bose explains curvature reversal by excitation conducted to the far side, a different mechanism from Loeb's local contraction.
The program notes that Loeb and Jennings disagree about tropisms.
Where it is discussedPassages matching symmetrical elements struck by light, photochemical effects, heliotropic curvature, positive heliotropism
1906Plant Response as a Means of Physiological Investigation · Bose, J. C.60
1918Forced Movements, Tropisms, and Animal Conduct · Loeb, J.48
1906The Dynamics of Living Matter · Loeb, J.44
1912The Mechanistic Conception of Life · Loeb, J.30
1916The Organism as a Whole, from a Physicochemical Viewpoint · Loeb, J.24
1900Comparative Physiology of the Brain and Comparative Psychology · Loeb, J.11
1926The Nervous Mechanism of Plants · Bose, J. C.9
1927Plant Autographs and Their Revelations · Bose, J. C.4
1901Regeneration · Morgan, T. H.3
1880The Power of Movement in Plants · Darwin, C.2