10 MR. DARWIN OK CLIMBING PLANTS. 



spiral twining of plants maintain that such plants have a natural 

 tendency to grow spirally. Mohl believes (S. 112) that twining 

 stems have a dull kind of irritability, so that they bond towards 

 anv object which they touch. Even before reading Mold's in- 

 teresting treatise, this view seemed to me so probable that 1 

 tested it in every way that I could, but always with negative 

 results. I rubbed many shoots much harder than is necessary to 

 excite movement in any tendril or in any foot-stalk of a leaf- 

 climber, but without result. I then tied a very light forked twig 

 to a shoot of a Hop, a Geropegia, Sphwoetema, and Adha&oda, 

 so that the fork pressed on one side alone of the shoot and re- 

 volved with it; 1 purposely selected some very slow revolvers, as 

 it seemed most likely that these would proiit from possessing irri- 

 tability ; but in no case was any etl'ect produced. Moreover, 

 when a shoot winds round a support, the movement is always 

 slower, as we shall immediately sec, than whilst its 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 

 be superfluous. Nevertheless 1 do not wish to assert that they 

 arc never irritable ; for the growing axis of the leaf-climbing, but 

 not spirally twining, Lophospermum scandens is, as we shall here- 

 after see, certainly irritable ; but this case gives mo confidence 

 that ordinary twiners do not possess this quality, for directly after 

 putting a stick to the Lophospermum, I saw that it behaved 

 differently from any true twiner or any other leaf-climber. 



The belief that twiners have a natural tendency to grow spirally 

 probably arose from their assuming this form when wound round 

 a support, and from the extremity, even whilst remaining free, 

 sometimes assuming this same form. The free internodes of 

 vigorously growing plants, when they cease to revolve, become 

 straight, and show no tendency to be spiral ; but when any shoot 

 has nearly ceased to grow, or when the plant is unhealthy, the 

 extremity does occasionally become spiral. I have seen this in a 

 remarkable degree with the ends of the shoots of the Statmtonia 

 and of the allied Akebia, which became closely wound up spirally, 

 just like a tendril, especially after the small, ill-formed leaves had 

 perished. The explanation of this fact is, I believe, that the 

 lower parts of such terminal internodes very graduallv and 

 successively lose their power of movement, whilst the portions 

 just above move onwards, and in their turn become motionless; 

 and this ends in forming an irregular spire. 



