i88i.] IN PLANTS. 337 



of this lower part, as I conclude from my experiments with 

 opaque caps, is influenced by the action of light on the upper 

 part. My opinion, however, on the above and many other 

 points, signifies very little, for I have no doubt that your book 

 will convince most botanists that I am wrong in all the points 

 on which we differ. 



Independently of the question of transmission, my mind is 

 so full of facts leading me to believe that light, gravity, &c., 

 act not in a direct manner on growth, but as stimuli, that I 

 am quite unable to modify my judgment on this head. I 

 could not understand the passage at p. 78, until I consulted 

 my son George, who is a mathematician. He supposes that 

 your objection is founded on the diffused light from the lamp 

 illuminating both sides of the object, and not being reduced, 

 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 



VOL. III. Z 



