CLIMBING PLANTS. 



bourliood, this contact occurs at the very beginning of circumnutation, 1)ut when 

 there are no erect stems close by, the shoot in its search liends its apex to all the 

 points of the compass, and describes wider and wider circles with its increasing 

 length. If in the space so traversed it finds no suitable support, the lower 

 portion of the shoot falls on the ground and becomes a procumbent stem ; but the 

 middle portion again rises up, and the free end twists round in a circle afresh. The 

 place where the nutation now occurs is removed some distance away from the spot 

 where it first began, and perhaps the revolving shoot in its new position may strike 

 against something which may serve it as a support. But if here also no suitable 

 support is encountered, a further migration may occur; thus a comparatively exten- 

 sive area is explored by the circling shoot in its quest for something to climb 

 around. The phenomena just detailed gave rise to the view formerly held, that 

 twining plants possessed the power of searching for a support, indeed, the idea was 

 favoured that the twining stem was positively attracted by such support. But such 

 a notion is disposed of by the actual facts. The meeting of the nutating shoot 

 with an erect stem must be held to be quite accidental, still it is certain that this 

 meeting is facilitated by the movements described above, and the probability of an 

 erect stem being encountered is obviously greater the more extensive the space 

 traversed by the shoot-apex. 



As soon as the revolving end of the shoot comes into contact with an erect 

 support of suitable thickness, it embraces the support, and adhering to it, twists 

 round it spirally and assumes the form of an elongated spiral wound around it. 

 This process may be illustrated by comparing it with the movement of a rope 

 swung in a circle coming in contact with a post, that is to saj', when one swings a 

 long rope or a long whip horizontally with the hands i-aised above the head, and at 

 the same time approaches so near to an erect post that the revolving rope must 

 reach it, then that portion of the rope beyond the point of contact twines spirally 

 round the post. 



It has been shown by manifold observations and experiments that erect props 

 are most easily embx'aced by twining stems. When the inclination of the prop 

 amounts to not less than 45° with the horizon, the twining shoot still forms a spiral 

 round it: but horizontal sticks are very seldom, though occasionally, entwined. It 

 has also been ascertained that the revolutions made by the twining stem become 

 both higher and steeper with increasing age. The coils formed by tlie youngest 

 and uppermost portions of the shoot are often very close together and almost 

 horizontal, but lower down the spii-al appears more di'awn out, and the newly- 

 formed upper flat coils are gradually pushed passively upwards. Thus the advan- 

 tage is obtained that the lower portion, as it assumes a steeper position, gets a better 

 grip of the support. In most cases of twining the stem is to some extent twisted 

 on itself, i.e. undergoes torsion. This torsion of the axis must not be confused with 

 its twisting around the support. The two things are distinct. We can take two 

 I'opes, in one of which the strands are twisted, whilst in the other they are straight. 

 Each of these may be wound i-ound a support. The former (i.e. the twisted rope) 



