16 TWINING PLANTS. Chap. 1 



a dull kind of irritability, so that they bend towards 

 any object which they touch ; but this is denied 

 by Palm. Even before reading Mold'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 Coropegia, Sphserostema, 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) by a 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 scribed. 



