TWINERS AND CLIMBERS. 189 



youngest, was 2\ inches. Therefore, the revolving 

 apex of this stem was about 14 inches in length, 

 and it swept round in a circle of 19 inches in diame- 

 ter, so that the rate of motion must have been but 

 little less than an inch in two minutes and a half, or 

 23 inches in an hour. 



The purpose of this rotation is so self-evident that 

 it scarcely needs explanation. It is undoubtedly 

 directed primarily in search of 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 project- 

 ing 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 sup- 

 port. 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 

 likewise loses its power of spirally twining." l 



1 Darwin, "The Movements of Climbing Plants,'' p. 15. 



