To G. J. Romanes   18 April 1881

Down, | Beckenham, Kent. | (Railway Station | Orpington. S.E.R.)

Apr 18th 1881.

My dear Romanes

I am extremely glad of your success with the flashing light. If plants are acted on by light like some of the lower animals, there is an additional point of interest, as it seems to me, in your results.1 Most botanists believe that light causes a plant to bend to it in as direct a manner as light affects nitrate of silver. I believe that it merely tells the plant to which side to bend; & I see indications of this belief prevailing even with Sachs.2 Now it might be expected that light wd act on a plant in something the same manner as on the lower animals.— As you are at work on this subject, I will call your attention to another point. Wiesner of Vienna (who has lately published a great book on Heliotropism) finds that an intermittent light during 20 m. produces same effect as a continuous light of same brilliancy during 60 m. So that van Tiegham in the first Part of his book which has just appeared, remarks, the light during 40 m. out of the 60 m. produced no effect.— I observed an analogous case described in my book.3

Wiesner & Tiegham seem to think that this is explained by calling the whole process “induction”, borrowing a term used by some Physico-chemists (of whom I believe Roscoe is one) & implying an agency which does not produce any effect for some time & continues its effect for some time after the cause has ceased.—4 I believe (?) that photographic paper is an instance. I must ask Leonard, whether an interrupted light acts on it in the same manner as on a plant.5 At present I must still believe in my explanation that it is the contrast between light & darkness which excites a plant.—

I have forgotten my main object in writing, viz to say that I believe (& have so stated) that seedlings vary much in their sensitiveness to light; but I did not prove this, for there are many difficulties,—Whether time of incipient curvature or amount of curvature is taken as the criterion.— Moreover they vary according to age & perhaps from vigour of growth; & there seems inherent variability as Strasburger (whom I quote) found with spores.—6 If the curious anomaly observed by you is due to varying sensitiveness, ought not all the seedlings to bend if the flashes were at longer intervals of time? According to my notion of contrast between light & darkness being the stimulus, I shd expect that if flashes were made sufficiently slow it would be a powerful stimulus, & that you would suddenly arrive at a period when the result would suddenly become great.— On the other hand, as far as my experience, goes, what one expects rarely happens.—

I heartily wish you success & remain | yours ever very sincerely | Ch. Darwin

I am extremely glad that you seem to have silenced Butler & his reviewers. But Mr Butler will turn up again, if I know the man.7 Do you read the Times? As I had a fair opportunity I sent a letter to Times on Vivisection, which is printed today.— I thought it fair to bear my share of the abuse poured in so atrocious a manner on all physiologists.—8

2
Nitrate of silver (now more commonly referred to as silver nitrate (AgNO3)) turns black when exposed to light through a process of photoreduction, forming silver oxide and nitric acid. CD may have formed his opinion of Julius Sachs’s view about the effect of light on plant organs from information from his son Francis Darwin. While Francis was working in Sachs’s laboratory, he had told CD that Sachs considered Julius Wiesner’s view of heliotropism ‘old-fashioned’ (see Correspondence vol. 27, letter from Francis Darwin, 29 May 1879).
3
In Movement in plants, p. 457, CD referred to his experimental results with Phalaris canariensis after cotyledons were briefly exposed to light several times; he cited Julius Wiesner’s observations on plants exposed to intermittent light in Wiesner 1878–80, p. 12. Philippe van Tieghem’s Traité de botanique (Tieghem 1884) was published in parts between 1881 and 1884; the first four parts are in the Darwin Library–CUL. CD refers to Tieghem 1884, p. 128.
4
Tieghem had referred to the concept of photochemical induction, elucidated by Robert Bunsen and Henry Enfield Roscoe (Bunsen and Roscoe 1855–9), in Tieghem 1884, p. 128 n. 1.
5
CD’s son Leonard Darwin was an instructor in chemistry and photography at the School of Military Engineering, Chatham.
6
In Movement in plants, p. 488, CD had referred to Eduard Strasburger’s paper on the movement of spores towards lateral light (Strasburger 1878a).
7
Romanes had written a highly critical review of Samuel Butler’s Unconscious memory (Butler 1880) in Nature, 27 January 1881, pp. 285–7, followed by a letter dismissing Butler’s response to the review (Nature, 10 February 1881, pp. 335–6). Butler had made accusations against CD and Ernst Krause (see letter to G. J. Romanes, 11 February 1881 and n. 1).
8
CD’s letter to Frithiof Holmgren, [14] April 1881, was published in The Times, 18 April 1881, p. 10.

Manuscript Alterations and Comments

0.1 (Railway … S.E.R.)] parentheses added
1.3 that light] after interl & del ‘&’
1.5 to which] after del ‘ho’
1.7 As] over illeg
1.9 light] interl
1.12 I … book. 1.13] interl
2.3 any] above del ‘its’
2.5 (?)] interl
2.7 contrast] above del ‘contrast’
3.1 (& … that 3.2] interl
3.2 light] above del ‘photo’
3.7 bend] above del ‘bend’
3.9 were made] interl
3.10 would] above del ‘would’
5.1 am] above del ‘am’
5.2 turn] above del ‘turn’
5.3 printed] above del ‘printed’

Please cite as “DCP-LETT-13124,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-13124