508 LECTURE XIX. 



What, now, are the causes of this twining ? It is clear, 

 since von Mohl and Palm found that shoots will not twine 

 round a horizontal support, an observation confirmed by 

 Schwendener's, that when a shoot and its support are rotated 

 on a clinostat no twining takes place, that the negative geo- 

 tropism of the shoots is an important factor in producing 

 twining. But the negative geotropism of these shoots can- 

 not, however, be the sole cause of their twining, for then any 

 excessively elongated etiolated shoot might twine. The cir- 

 cumnutation of the shoot, for instance, may contribute to the 

 twining, and it is possible to conceive, as von Mohl did, that 

 the shoot is sensitive to contact. 



With regard to the possible connexion between circumnu- 

 tation and twining, it must be pointed out that in all cases 

 the direction of twining is the same as that of circumnutation. 

 In consequence of this, circumnutation has come to be 

 regarded as the primary cause of twining, but its mode of 

 action has been differently regarded by various observers. 

 Palm first suggested a view with regard to its mode of 

 action which Darwin subsequently stated with great pre- 

 cision. He says, "When a revolving shoot meets with a 

 support, its motion is necessarily arrested at the point of 

 contact, but the free projecting part goes on revolving. 

 As this continues, higher and higher points are brought into 

 contact with the support and are arrested ; and so onwards to 

 the extremity; and thus the shoot winds round its support." 

 And again : " If a man swings a rope round his head, and the 

 end of it hits a stick, it will coil round the stick according to 

 the direction of the swinging movement; so it is with a 

 twining plant, a line of growth travelling round the free part 

 of the shoot causing it to bend towards the opposite side, 

 and this replaces the momentum of the free end of the rope." 

 This very simple explanation is, however, insufficient. In the 

 first place, in the circumnutation of the end of a shoot no 

 momentum is acquired which might be compared to the 

 centrifugal force in the swinging rope ; secondly, the axis of 

 circumnutation does not necessarily coincide with the axis of 

 twining, as is the case in the swinging rope ; thirdly, the 



