TOWER OF MOVEMENT IN PLANTS.' [1881 



with increasing distance in the same ratio as the direct light ; 

 but he doubts whether this necessary correction will account 

 for the very little difference in the heliotropic curvature of 

 the plants in the successive pots. 



With respect to the sensitiveness of the tips of roots to 

 contact, I cannot admit your view until it is proved that I am 

 in error about bits of card attached by liquid gum causing 

 movement ; whereas no movement was caused if the card 

 remained separated from the tip by a layer of the liquid gum. 

 The fact also of thicker and thinner bits of card attached on 

 opposite sides of the same root by shellac, causing movement 

 in one direction, has to be explained. You often speak of 

 the tip having been injured ; but externally there was no sign 

 of injury : and when the tip was plainly injured, the extreme 

 part became curved towards the injured side. I can no more 

 believe that the tip was injured by the bits of card, at least 

 when attached by gum- water, than that the glands of Drosera 

 are injured by a particle of thread or hair placed on it, or that 

 the human tongue [is so] when it feels any such object. 



About the most important subject in my book, namely 

 circumnutation, I can only say that I feel utterly bewildered 

 at the difference in our conclusions ; but I could not fully 

 understand some parts which my son Francis will be able to 

 translate to me when he returns home. The greater part of 

 your book is beautifully clear. 



Finally, I wish that I had enough strength and spirit to 

 commence a fresh set of experiments, and publish the results, 

 with a full recantation of my errors when convinced of them ; 

 but I am too old for such an undertaking, nor do I suppose 

 that I shall be able to do much, or any more, original work. 

 I imagine that I see one possible source of error in your 

 beautiful experiment of a plant rotating and exposed to a 

 lateral light. 



With high respect and with sincere thanks for the kind 

 manner in which you have treated me and my mistakes, I 

 remain, my dear Sir, yours sincerely, 



CHARLES DARWIN. 



