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 w^d 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 sh^d 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 M^r 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