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# From Francis Darwin   [22 June 1878]1

Hotel de Russie | Würzburg

Saturday

My dear Father,

I have had a talk with Sachs about oat & canary grass   he says the chlorophyll is only beginning to be developed when the seedling appears above ground & that the assimilation is extremely small for the first few days— He evidently thought that assimilation was practically not going on for the first few days. If you want to know more accurately I can easily grow some oats, & it would be good practice in section cutting or perhaps in spectroscoping.2 But Sachs seemed very confident about the complete unimportance of assimilation. I have borrowed Cieselski & read him.3 Sachs doesn’t consider that there is any puzzle as to how the difference between their results arose. He says the bend up only occurs when the root has previously lost water & & then the cells on the water surface become more turgid & cause the upward bend. I think I only saw a passage about Cieselski in ‘Ablenkung der Wurfeln &’ p 219— I suppose you have seen the place in “Ueber das Wachsthum der Hapt u Nebenw” p 401, where he talks about the fact that the roots must be slightly dry for C’s bend.4 I have read most of Cieslski & I dont think there is much to trouble about. Sachs doesn’t think much of him, although some of his experiments were good. One fact of his would be interesting if it were true but Sachs says it is “falsch’— If you cut the extreme tip of a root, it goes on growing pretty well though the punctum vegetationis is destroyed. Cieselski says that while this growth continues & before a new punctum vegis. is formed that the roots does not care for gravity & will grow in any direction it is placed in5  This Sachs says is not true.

Cieselski says if you shave a long sloping bit off a root—& place the root horizontally then if the cut surface is downwards the root will bend down geotropically even quicker than usual, but if the cut surface is uppermost, the root will sometimes bend up before it bends down. This looks like bending towards the cut side.

I have got a most queer looking sleeper—? hygrometrica: it has pinnate leaves like M. Farnesiana & Sachs says it always asleep—he doesn’t understand the movements: he thinks it must be suited for some very exceptional climate or position: it has a great deal of wood & very few leaves & all afternoon in shade they were pressed back agains the stems & the leaflets shut—I will observe it.6 Sachs declares he believes that one use of sleeping plants shutting up on being irritated is a protection against hail. There was a summer hailstorm here of only 5 or 10 minutes which did many pounds worth of damage by killing plants, & some mimosas were out in it & were quite uninjured   He thinks the flaccidity of the leaf in the irritated state important as making it yield on being struck. He seemed quite inclined to believe in protection from cold. Also in bloom being protection against dryness. He thinks blo〈om〉 must have many functions & said it was a protection against insects.7 He had never noticed the fact that when you wet the lower end of a yew stick water appears above & quite agrees with my idea that it is important, he evidently didint much believe in it till he tried it himself & then was pleased with it

I have not got any good Hydrocharis plants so have not begun.8 I have been cutting to get exactly central ones. I think Sachs thought I wanted to publish something & that was why he gave me the the Hydrocharis to do, I told him that I would rather do something that hadn’t been finished even tho’ it led to nothing so I have started trying to make out what changes in strength & in elasticity go on as a woody stem loses water

At present the experiments are very simple

A stick or grass stalk fixed horizontally at one end & a weight to take off & put on, & I measured the movement with my mercury microm9   The ultimate object is to know what share the tension of the cell walls (not the hydrostatic turgor) takes in causing the stiffness of a stem. One ought to be a first rate mathematician for such a thing. I shall also do some experiments putting fresh & withered shoots into alcohol & seeing what happens. Sachs has done some experiments, & says they both retain their form very nearly, but he doesn’t know exactly. Sachs told me of a real fools experiment he tried, he thought the green colour of cabbage catterpillars must come from chlorophyll & so he tried feeding them on etiolated leaves—but they would not turn white.10 I think it would be a good thing to have a telescope like Vines used, it would do for any careful observations—they are made in Munich & I could order one like Sachs’   they cost about £10.11 I should think there are at least 50 thermometers in the laboratory   some beauties divided in $\frac{1}{10}$th Centigrade. The fittings generally are very simple; he has a great many big cylinders of glass & all sorts of zinc covers & boxes for various things—& splendid lots of cork, also silvered wire for fastening things under water, tho’ brass does as well I should think.

My love to everybody | Yr affec son | Frank Darwin

## CD annotations

3.1 I have got … M. Farnesiana 3.2] double scored red crayon
6.13 I … observations— 6.14] double scored pencil
6.15 at least … Centigrade. 6.16] cross in left margin, pencil

## Footnotes

The date is established by the relationship between this letter and the letter to Francis Darwin, 26 June [1878]. In 1878, the Saturday before 26 June was 22 June.
Francis was investigating whether there was chlorophyll in the cotyledons of canary grass (Phalaris canariensis) and oats (Avena sativa). Thin sections examined under a microscope could provide a view of individual cells containing chlorophyll. The spectroscope (invented in the late 1850s by Gustav Robert Kirchhoff and Robert Wilhelm Eberhard Bunsen) provided a means of calculating the absorption spectra of a chemical element or compound. In 1871, Kliment Arkadievich Timiryazev had published the first spectral analysis of chlorophyll (Timiryazev 1871), in which he had shown that absorption was restricted to blue and red wavelengths; his analysis could therefore be used to detect the presence of chlorophyll in cells. For contemporary views of what constituted the cotyledon of grasses, see the letter from George Henslow, [c. 20 February 1878] and n. 3; for CD’s own usage, see Movement in plants, p. 62. Francis was working in Julius Sachs’s laboratory in Würzburg, Germany, over the summer (see letter to W. T. Thiselton-Dyer, 2 June 1878 and n. 5).
CD had asked Francis to read Theophil Ciesielski’s observations on the movement of radicles (embryonic roots) when in contact with water (Ciesielski 1871, p. 33; see also Ciesielski 1872, p. 25). See letters to Francis Darwin, [13–26 May 1878] and 18 June [1878]).
In his paper ‘Ablenkung der Wurzel von ihrer normalen Wachsthumsrichtung durch feuchte Körper’ (Deflection of the root from its normal direction of growth by moisture; Sachs 1872, p. 219), Sachs observed that radicles turned towards water whereas Ciesielski’s experiments showed that roots laid on a horizontal wet surface grew upwards away from the moisture. In ‘Ueber das Wachsthum der Haupt- und Nebenwurzeln’ (On the growth of primary and adventitious roots), Sachs dismissed Ciesielski’s results, claiming they were due to Ciesielski’s having allowed the roots in his experiments to dry out partially during preparation (Sachs 1873–4, p. 401). CD had reported that his experiments on radicles were inconclusive (see letter to Francis Darwin, 18 June [1878]).
Falsch: wrong (German). Ciesielski 1871, pp. 29–30; see also Ciesielski 1872, pp. 21–2. The punctum vegetationis is the growing point.
CD had asked Francis to check sleeping plants in the botanic garden greenhouses at night (see letter to Francis Darwin, 18 June [1878]). Francis later identified the species as Porliera hygrometrica (letter from Francis Darwin, [29 June] 1878). In taxonomic literature, Porliera is considered an incorrect subsequent spelling of Porlieria; the name Porlieria hygrometrica is unresolved, but is likely to be an error for P. hygrometra; the name was sometimes applied to specimens later identified as P. chilensis (see, for example, Johnston 1938, pp. 253–4). Mimosa farnesiana is a synonym of Vachellia farnesiana (sweet acacia).
CD had begun studying bloom (the waxy or powdery coating on leaves and fruit) in August 1873 (see Correspondence vol. 21, letter to J. D. Hooker, 13 August 1873). He had resumed investigating the function of bloom on leaves in 1877 (see Correspondence vol. 25, letter to Fritz Müller, 14 May 1877 and n. 2).
Hydrocharis is a genus of aquatic plants.
A mercury micrometer is used to measure very small changes in pressure; a micron is one millionth part of a metre.
Cabbage caterpillars: either caterpillars of the small cabbage white butterfly, Pieris rapae, which are green, or those of the cabbage moth, Mamestra brassicae, which vary from green to brown or blackish.
Sydney Howard Vines had investigated the influence of light on unicellular organisms while working in Sachs’s laboratory; he described the ‘micro-telescope’ he used in Vines 1878, p. 135. The instrument had been made by C. A. Steinheil Söhne of Munich.

## Bibliography

Ciesielski, Theophil. 1871. Untersuchungen über die Abwärtskrümmung der Wurzel. Inaugural-Dissertation welche mit Genehmigung philosophischen Facultät der königl. Universität zu Breslau zur Erlangung der Doctorwürde. Breslau: R. Nischkowsky.

Ciesielski, Theophil. 1872. Untersuchungen über die Abwärtskrümmung der Wurzel. Beiträge zur Biologie der Pflanzen 1 (1870–5) Heft 2: 1–30.

Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 27 vols to date. Cambridge: Cambridge University Press. 1985–.

Johnston, Ivan M. 1938. New or noteworthy plants from temperate South America. Journal of the Arnold Arboretum 19: 248–63.

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

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

Timiryazev, Kliment Arkadievich. 1871. Spektralny analiz khlorofilla. St Petersburg: Press of the Society for Social Benefit.

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.

## Summary

Describes his talk with Julius von Sachs about canary-grass.

## Letter details

Letter no.
DCP-LETT-12131F
From
Francis Darwin
To
Charles Robert Darwin
Sent from
Hotel de Russie, Würzburg
Source of text
DAR 274.1: 51

## Please cite as

Darwin Correspondence Project, “Letter no. 12131F,” accessed on 14 April 2021, https://www.darwinproject.ac.uk/letter/DCP-LETT-12131F.xml

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