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Darwin Correspondence Project

From Francis Darwin   [before 26 June 1879]1

Bot Institut | Würzburg

My dear Father,

Never mind about Rhagadiolus & Hieracium, I am sorry I bothered you, as I have managed to get some seeds here.2 I did the caustic experiment with Faba & Phaseolus in damp earth & by evening they had all grown well & the caustic ones had not bent, but next morning many of the caustic ones were bent—so Sachs doesn’t believe in it a bit;3 he says the growth is disturbed and that anything that disturbs growth prevents geotropism— as for instance merely growing in damp air in some cases. He also says caustic is not a proper thing for the work because nitric acid will be set free & this will be diffused back into the root & injure it!!!!4 It is certain that the true growing cells of the root itself under the root-cap are killed (Göbel5 looked at the caustic sections & said so) but one could not cause so slight an injury with a razor, which Sachs thinks would be better, because even if you could confine your slice to the cells so near the surface the water in which the roots would be put to grow in would cause a deeper injury: but with caustic you injure the surface & the injury spreads only a little way in. Sachs doesn’t believe that in the shellac experiment it is the touching of the little bit of shellacced-glass that makes the root bend, because he says in his experiments diagram with vessels with sloping walls the root grows down as (a) whereas if the touching had any effect it ought to go as b— Of couse the thing is that the bending ceases directly the root has bent away. I said you had done experiments with smoked glass and that the roots grew down sloping surfaces not by pressing hard against them, but only touching in a number of places or at least touching very lightly.6 Then he said that the smoke may cause injury to the root! one feels inclined to say— If you say that its no use talking with you. I thought you had observed that a root growing down a clean glass slide seemed to go along without touching the glass, but I wasn’t sure enough to say so.

I have started some heliotropic caustic experiments but: the roots even without caustic bend so badly in these pitch dark green houses that it is no good I am afraid. I have today started another in better light. I did one pot of mustard roots but alas the caustic ones were apheliotropic. The mustard roots are extraordinarily sensitive. I put some in the middle of this big laboratory in a black box whose opening was covered with thickish writing paper & put it facing the N on a cloudy day & they were clearly bent from light: the cotyledon had not light enough to be properly green

My great difficulty is making the marks. I do it with aspalt varnish. & put the roots on wet blotting paper under a bell for a few minutes while the varnish dries, but this amount of drying seems to hurt the roots, as some do not grow or hardly so,— (but these are only 2 or 3 out of 20 or 30): sometimes I can get the varnish so dry that I can pop the roots strt into water & then they grow much better. If I can get a bit of clockwork put to rights I shall measure them with a telescope like Vines did.7 I shan’t bother about Rhizomorpha as the seedling roots will decide the question.8

My love to Mother   I will write a decent letter next time | Your affec F.D.

CD annotations

Top of letter: ‘Mustard | on floating cork’9 ink


The date is established by the relationship between this letter and the letter to Francis Darwin, 28 June [1879], in which CD mentioned receiving this letter just before setting off for London. The Darwins left Down on 26 June 1879 (Emma Darwin’s diary (DAR242)).
Francis’s request for seeds of Rhagadiolus (a genus in the daisy family, Asteraceae) and Hieracium (the genus of hawkweed) was made on a now missing postcard (see letter to Francis Darwin, [26 June 1879]).
CD was studying the sensitivity of the apex of the radicle (embryonic root) and had recently begun a series of experiments with caustic to determine how geotropism in the radicle was controlled (see letter to Francis Darwin, [before 5 June 1879]). Francis was working in the laboratory of Julius Sachs in Würzburg; Sachs had also investigated influences on root movement (Sachs 1872a and 1873–4). Phaseolus (wild bean) and Vicia (vetch) are genera in the family Fabaceae (legumes).
Lunar caustic is silver nitrate (AgNO3); nitric acid is HNO3.
CD had used sloping smoked-glass plates to study the movement of radicles of various plants and concluded that the tips of the radicle circumnutated as they moved downwards, leaving serpentine trails along the glass (see Movement in plants, pp. 28–31).
Sydney Howard Vines had used a micro-telescope developed by Georg Quincke to measure the growth of unicellular organs. Vines’s specimens were rotated by a clockwork mechanism in order to avoid heliotropic curvature when exposed to light (Vines 1878, pp. 134–5; see p. 135 for a diagram of the apparatus).
Rhizomorphs are root-like aggregations of the hyphae or branching filaments that make up the mycelium in fungi.
CD’s annotation is a note for his reply (see first letter to Francis Darwin, 2 July [1879]).


Movement in plants: The power of movement in plants. By Charles Darwin. Assisted by Francis Darwin. London: John Murray. 1880.

Sachs, Julius. 1872a. Ablenkung der Wurzeln von ihrer normalen Wachsthumsrichtung durch feuchte Körper. Arbeiten des Botanischen Instituts in Würzburg 1 (1871–4): 209–22.

Sachs, Julius. 1873–4. Ueber das Wachsthum der Haupt- und Nebenwurzeln. Arbeiten des Botanischen Instituts in Würzburg 1 (1871–4): 385–474, 584–634.

Vines, Sydney Howard. 1878. The influence of light upon the growth of unicellular organs. Arbeiten des botanischen Instituts in Würzburg 2 (1878–82): 133–47.


Discusses results of geotropism experiment. Has started some heliotropic caustic experiments on mustard roots. Has trouble making marks.

Letter details

Letter no.
Francis Darwin
Charles Robert Darwin
Sent from
Source of text
DAR 274.1: 55
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 12126F,” accessed on 21 April 2024,