16 TWINING PLANTS. Cuar. I, 
a dull kind of irritability, so that they bend towards 
any object which 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 
anegative 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 Coropegia, Sphexrostema, 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.* 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 
* Dr. H. de Vries also has plants are not irritable, and that 
shown (ibid. p. 321 and 325) bya the cause of their winding up a 
better method than that employed support is exactly what I have de- 
by me, that the stems of twining  gcribed. 
