390 The Movements of Plants 



wool fibre was disturbed by the wind. The same author expanded 

 and explained in a most interesting way the meaning of Darwin's 

 observation that tendrils are not stimulated to movement by drops 

 of water resting on them. Pfeffer showed that dirty water contain- 

 ing minute particles of clay in suspension acts as a stimulus. He 

 also showed that gelatine acts like pure Avater ; if a smooth glass rod 

 is coated with a 10 per cent, solution of gelatine and is then applied 

 to a tendril, no movement occurs in spite of the fact that the gelatine 

 is solid when cold. Pfeffer^ generalises the result in the statement 

 that the tendril has a special fonn of irritability and only reacts to 

 "differences of pressure or variations of pressure in contiguous... 

 regions." Darwin was especially interested in such cases of specialised 

 irritability. For instance in May, 1864, he wrote to Asa Gray^ 

 describing the tendrils of Dlgnonia capreolata, which "abhor a 

 simple stick, do not much relish rough bark, but delight in wool 

 or moss." He received, from Gray, information as to the natural 

 habitat of the species, and finally concluded that the tendrils "are 

 specially adapted to climb trees clothed with lichens, mosses, or other 

 such productions V 



Tendrils were not the only instance discovered by Darwin of 

 delicacy of touch in plants. In 1860 he had already begun to observe 

 Sundew {Drosera), and was full of astonishment at its behaviour. 

 He wrote to Sir Joseph Hooker*: "I have been working like a 

 madman at Drosera. Here is a fact for you which is certain as you 

 stand M here you are, though you won't believe it, that a bit of hair 

 7T5^ of one grain in weight placed on gland, will cause one of the 

 gland-bearing hairs of Drosera to curve inwards." Here again 

 Pfeffer^ has, as in so many cases, added important facts to my father's 

 observations. He showed that if the leaf of Drosera is entirely freed 

 from such vibrations as would reach it if observed on an ordinary 

 table, it does not react to small weights, so that in fact it was the 

 vibration of the minute fragment of hair on the gland that produced 

 movement. We may fancifully see an adaptation to the capture 

 of insects — to the dancing of a gnat's foot on the sensitive surface. 



Darwin was fond of telling how when he demonstrated the 

 sensitiveness of Drosera to Mr Huxley and (I think) to Sir John 

 Burdon Sanderson, he could perceive (in spite of their courtesy) that 



^ Phjsiolofjy, Eng. Tr. ni. p. 52. Pfeffer has pointed out the resemblance between the 

 contact irritability of plants and the human sense of touch. Our skin is not sensitive to 

 nniform pressure such as is produced when the finger is dipped into mercury (Tiibingeu 

 TJnterxuchungen, i. p. 504). 



'•* Life and Letters, in. p. 314. 



» Climhing Plants, p. 102. 



* Lift" and Letters, in. p. 319. 



' Pfeffer in Untersuchungen a. d. Dot. Inst. z. Tubingen, i. p. 491. 



