nts", page 102.)

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 ("Life and Letters", III. page 319.): "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 1/78000 of one grain in weight placed on gland, will cause
ONE of the gland-bearing hairs of Drosera to curve inwards." Here again
Pfeffer (Pfeffer in "Untersuchungen a. d. Bot. Inst. z. Tubingen",
I. page 491.) 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 they thought the whole
thing a delusion. And the story ended with his triumph when Mr Huxley
cried out, "It IS moving."

Darwin's work on tendrils has led to some interesting investigations on
the mechanisms by which plants perceive stimuli. Thus Pfeffer (Tubingen
"Untersuchungen" I. page 524.) showed that certain epidermic cells
occurring in tendrils are probably organs of touch. In these cells the
protoplasm burrows as it were into cavities in the thickness of the
external cell-walls and thus comes close to the surface, being separated
from an object touching the tendril merely by a very thin layer of
cell-wall substance. Haberlandt ("Physiologische Pflanzenanatomie",
Edition III. Leipzig, 1904. "Sinnesorgane im Pflanzenreich", Leipzig,
1901, and other publications.) has greatly extended our knowledge of
vegetable structure in relation to mechanical stimulation. He defines a
sense-organ as a contrivance by which the DEFORMATION or forcible change
of form in the protoplasm--on which mechanical stimulation depends--is
rendered rapid and considerable in amplitude ("Sinnesorgane", page 10).
He has shown that in certain papillose an