the same time say with
Pfeffer that "the causes of twining are... unknown." ("The Physiology of
Plants", Eng. Tr. (Oxford, 1906), III. page 37.)

Let us leave this difficult question and consider some other points made
out in the progress of the work on climbing plants. One result of what
he called his "niggling" ("Life and Letters", III. page 312.) work on
tendrils was the discovery of the delicacy of their sense of touch, and
the rapidity of their movement. Thus in a passion-flower tendril, a bit
of platinum wire weighing 1.2 mg. produced curvature ("Climbing
Plants", page 171.), as did a loop of cotton weighing 2 mg. Pfeffer
("Untersuchungen a.d. Bot. Inst. z. Tubingen", Bd. I. 1881-85, page
506.), however, subsequently found much greater sensitiveness: thus
the tendril of Sicyos angulatus reacted to 0.00025 mg., but this only
occurred when the delicate rider of cottonwool 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 containing 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 ("Physiology", Eng. Tr. III.
page 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 (Tubingen "Untersuchungen", I. page 504.)
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 ("Life and Letters", III. page 314.)
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."
("Climbing Pla