i868 1881] HELIOTROPISM 431 



To G. J. Romanes. Letter 762 



Down, April i8th. 1881. 



I am extremely glad of your success with the flashing 

 light. 1 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. 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, 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 great book 2 on helio- 

 tropism) finds that an intermittent light, say of 20 minutes, 

 produces the same effect as a continuous light of, say 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. 3 



Wiesner and Van 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 pro- 

 duce any effect for some time, and continues its effect for 

 some time after the cause has ceased. I believe that photo- 

 graphic paper is an instance. I must ask Leonard 4 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 the time of incipient 



1 Romanes' paper on the effect of intermittent light on heliotropism 

 was the Proc. Royal Soc., Vol. LIV., p. 333. 



2 Wiesner's papers on heliotropism are in the Denkschriften of the 

 Vienna Academy, Vols. 39 and 43. 



Power of Movement, p. 459. 

 4 Mr. Darwin's son. 



