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 water ; 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 form 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 Bignonia 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®.” 
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 where you are, though you won't believe it, that a bit of hair 
rstoo 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 
1 Physiology, Eng. Tr. m1. 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 
uniform pressure such as is produced when the finger is dipped into mercury (Tibingen 
Untersuchungen, 1. p. 504). 
2 Life and Letters, m1. p. 314. 
3 Climbing Plants, p. 102. 
4 Life and Letters, m1. p. 319. 
5 Pfeffer in Untersuchungen a. d. Bot. Inst. z. Tubingen, 1. p. 491, 
