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.