Caap. I. TWINING PLANTS. 15 
tinuous bowing movement 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 
increasesin 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 
internode 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 Palm 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 
