they touch; but this is denied by Palm. Even before reading Mohl's
interesting treatise, this view seemed to me so probable that I
tested it in every way that I could, but always with a negative
result. 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.
{12}

The belief that twiners have a natural tendency to grow spirally,
probably arose from their assuming a spiral form when wound round a
support, and from the extremity, even whilst remaining free,
sometimes assuming this form. The free internodes of vigorously
growing plants, when they cease to revolve, become straight, and show
no tendency to be spiral; but when a shoot has nearly ceased to grow,