I rubbed many shoots much harder than is necessary to excite
movement in any tendril or in the foot-stalk of any leaf climber, but
without any effect. I then tied a light forked twig to a shoot of a
Hop, a Ceropegia, Sphaerostemma, and Adhatoda, so that the fork
pressed on one side alone of the shoot and revolved with it; I
purposely selected some very slow revolvers, as it seemed most likely
that these would profit most from possessing irritability; but in no
case was any effect produced. {11} Moreover, when a shoot winds
round a support, the winding movement is always slower, as we shall
immediately see, than whilst it revolves freely and touches nothing.
Hence I conclude that twining stems are not irritable; and indeed it
is not probable that they should be so, as nature always economizes
her means, and irritability would have been superfluous.
Nevertheless I do not wish to assert that they are never irritable;
for the growing axis of the leaf-climbing, but not spirally twining,
Lophospermum scandens is, certainly irritable; but this case gives me
confidence that ordinary twiners do not possess any such quality, for
directly after putting a stick to the Lophopermum, I saw that it
behaved differently from a true twiner or any other leaf-climber.
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