Chap. I. TWINING PLANTS. 15 



tinuous boAving moTement directed successively to all 

 points of the compass, is, as Mohl has remarked, to 

 favour the shoot finding a support. This is admirably 

 effected by the revolutions carried on night and day, 

 a wider and wider circle being swept as the shoot 

 increases in length. This movement likewise explains 

 how the plants twine ; for 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 ex- 

 tremity ; and thus the shoot winds round its support. 

 When the shoot follows the sun in its revolving 

 course, it winds round the support from right to left, 

 the support being supposed to stand in front of the 

 beholder ; when the shoot revolves in an opposite 

 direction, the line of winding is reversed. As each 

 intemode loses from age its power of revolving, it like- 

 wise loses its power of spirally twining. If a man 

 swings a rope round his head, and the end 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. 

 All the authors, except Pabn and Mohl, who have 

 discussed the spiral twining of plants, maintain that 

 such plants have a natural tendency to grow spirally. 

 Mohl believes (p. 112) that twining stems have 



