HOW PLANTS CLIMB. 201 



SO high as the level of our forest tree-tops, so they select smaller 

 and shorter supports in other situations. 



The main purpose of plants becoming climbers in any way is, 

 of course, to reach such a position as to enable them to expose 

 their leaves to the action of air and light, with as little expendi- 

 ture of matter as possible. In the case of twiners, their first step 

 is to fi7id some support on which they can rely, towards the 

 attainment of the end in view. It is in order to find such support 

 that the spontaneous revolving movement is carried on by day 

 and night, the shoot sweeping in wider and wider circles. This 

 shows us how the plant twines, for when a revolving shoot meets 

 with a support, this support of course arrests the movement at the 

 point of contact, while the free portion continues revolving. 

 Thus higher and higher parts are one by one arrested, and the 

 shoot winds round its support. Such is Darwin's own explanation. 



How is the revolving movement effected ? It was formerly 

 supposed that it v.-as wholly due to a twisting of the shoot or 

 stem on its own axis. This is now conclusively disproven, for 

 many plants clearly revolve, especially among leaf-climbers and 

 tendril-bearers, and yet their internodes are in no way twisted. 

 We also meet with instances where different internodes are 

 twisted in opposite ways, and even in an opposite direction to 

 that of their revolutions. The axial torsion seems rather to bear 

 relation to ruggedness or inequalities of the support, and to the 

 power of revolving freely without any support. 



Nevertheless, seeing that although many plants, not being 

 twiners, are axially twisted, and that this tendency is much 

 stronger and more frequent m plants that do twine, it is probable 

 that there is some relation between the power to twine and the 

 presence of their axial twisting. 



The revolving movement is effected as follows : — It is a 

 successive bowing over of the steni^ first in one direction, then in 

 another, and so on, until a circle has been completed — i.e., the 

 stem i?, pulled over, so to speak, by some internal force, acting in 

 turn all round the stem in the direction in which it is sweeping, so 

 that the circuit is made without any real twisting. This is not 

 easy to explain in words, but suppose we paint a dotted line along 

 the upper or convex side of a shoot bent towards the South. Let 



