I believe that it merely tells the plant to which side to bend, and I see indications of this belief prevailing even with Sachs. Now it might be expected that light would 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 good 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 Tieghem, 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. Wiesner and Tieghem 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), and implying an agency which does not produce any effect for some time, and continues its effect for some time after the cause has ceased. 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. At present I must still believe in my explanation that it is the contrast between light and darkness which excites a plant.
I have forgotten my main object in writing, viz. to say that I believe (and 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 and perhaps from vigour of growth; and there seems inherent variability, as Strasburger (whom I quote) found with spores. 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 and darkness being the stimulus, I should expect that if flashes were made sufficiently slow it would be a powerful stimulus, and 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, and remain, yours ever very sincerely,
Ch. Darwin.
Do you read the 'Times'? As I had a fair opportunity, I sent a letter to the 'Times' on Vivisection, which is printed to-day. I thought it fair to bear my share of the abuse poured in so atrocious a manner on all physiologists.
From G. J. Romanes to C. Darwin, Esq.
18 Cornwall Terrace: April 22.
I have left your last letter so long unanswered in order that I might be able to let you know the result of the next experiment I was trying on the seeds with flashing light. I think in the end the conclusion will be that short flashes, such as I am now using, influence the seedlings, but only to a comparatively small degree, so that it is only the more sensitive seedlings that perceive them.
Your letter in the 'Times' was in every way admirable, and coming from you will produce more effect than it could from anybody else. The answer to-day to —— is also first-rate—just enough without being too much. It would have been a great mistake to have descended into a controversy. I thought —— had more wit than to adopt such a tack and tone, and am sure that all physiologists will be for ever grateful to you for such a trenchant expression of opinion.
I have a little piece of gossip to tell. Yesterday the Council of the Linnean nominated me Zoological Secretary, and some of the members having pressed me to accept, I have accepted. I also hear that your son is to be on the same Council, and that Sir John Lubbock is to be the new President.