Chemotropism

In science, chemotropism, from the Egyptian chem­-, meaning “chemical” + “-tropism”, from the Greek trepein “to turn”, coined by Wilhelm Pfeffer (c.1893), in German (Chemotropismus), refers to the induced turning of a thing via chemical mediator, e.g. the male fern cells attracted to malic acid, or male moths to female moth pheromones.

Haeckel
In 1894, Ernst Haeckel, in his “The Phylogeny of the Plant Soul”, differentiated the various tropisms as follows: [1]

“All metaphyta are more or less sensitive to the influence of light (heliotropism), heat (thermotropism), gravity (geotropism), electricity (galvanotropism), and various chemical excitations (chemotropism). The quality and quantity of the sensation due to the irritation, as of the motor or trophic reaction produced by it, varies, however, exceedingly in the different groups of plants and frequently even in closely allied species of one genus or family. It is very small or hardly perceptible in many lower “sense-blunted” plants and especially in parasites.”

Herein, Haeckel, of note, attempts to anthropomorphize cells, e.g. speaking of "erotic chemotropism" and "social chemotropism", when referring to cellular reproduction.

In 1893 to 1903, Haeckel, in his love letters, mentions the concept of elective affinity at least three times in respect to his own romantic relationships. In one letter to a Franziska von Altenhausen, Haeckel defines elective affinity as a strange psychological chemotropism: [2]

“… seductive women—why should I, despite all scruples and obstacles, cast myself into the dust before you? Dearst Franziska, herein lies the enigma of ‘elective affinity’, of that strange psychological ‘chemotropism’, of whose power I have spoken repeatedly in my books—little dreaming that I myself should fall a victim to it in my old age!”

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Weininger
In 1903, Otto Weininger, in his Sex and Character, after citing (Ѻ) Haeckel’s work on “gonochorism”, as a the subject of separation or classification of the sexes, refers, firstly, to the c.1870s chemotropism experiments of Wilhelm Pfeffer as follows: [3]

“The experiments of Wilhelm Pfeffer have shown that the male cells of many cryptogams are naturally attracted not merely by the female cells, but also by substances which they have come in contact with under natural conditions, or which have been introduced to them experimentally, in the latter case the substances being sometimes of a kind with which they could not possibly have come in contact, except under the conditions of experiment. Thus the male cells of ferns are attracted not only by the malic acid secreted naturally by the archegonia, but by synthetically prepared malic acid, whilst the male cells of mosses are attracted either by the natural acid of the female cells or by acid prepared from cane sugar. A male cell, which, we know not how, is influenced by the degree of concentration of a solution, moves towards the most concentrated part of the fluid. Pfeffer named such movements "chemotactic " and coined the word "chemotropism" to include these and many other asexual cases of motion stimulated by chemical bodies. There is much to support the view that the attraction exercised by females on males which perceive them at a distance by sense organs is to be regarded as analogous in certain respects with chemotropism.”
— Otto Weininger (1903), Sex and Character (pg. 39)

“It is plain that too much stress must not be laid on the analogy between sexual affinity and purely chemical processes. None the less, I thought it illuminating to make the comparison. It is not yet quite clear if the sexual attraction is to be ranked with the "tropisms," and the matter cannot be settled without going beyond mere sexuality to discuss the general problem of erotics. The phenomena of love require a different treatment, and I shall return to them in the second part of this book. None the less, there are analogies that cannot be denied when human attractions and chemotropism are compared. I may refer as an instance to the relation between Edward and Ottilie in Goethe's Elective Affinities (Wahlverwandtschaften).”
— Otto Weininger (1903), Sex and Character (pgs. 42-43)

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References
1. Haeckel, Ernst. (1894). “The Phylogeny of the Plant Soul” (Ѻ), The Open Court, Volumes 8-9:4458-.
2. Haeckel, Ernst. (1930). The Love Letters of Ernst Haeckel: Written Between 1898 and 1903 (editor: Johannes Werner) (elective affinity, pgs. 101, 212, 260). Methuen.
3. Weininger, Otto. (1903). Eros and Psyche or Sex and Character: A Fundamental Investigation (Geschlecht und Charakter: Eine prinzipielle Untersuchung) (pgs. 39, 42-43). Vienna: Braumüller & Co, 1906.

Further reading
● Bunning, Erwin. (1989). Ahead of His Time, William Pfeffer: Early Advances in Plant Biology (chemotropism, pg. 43). MQUP.

External links
Chemotropism – Wikipedia.

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