CLIMBING PLANTS. 6+7 



that in this respect the power of spiral contraction gives a tendril- 

 climber an advantage over leaf-climbers which have no contracting 

 power, and therefore no means of hauling themselves up to supporting 

 objects. 



But the spiral contraction of tendrils has another use, and this is 

 probably the most important one. This use depends on the fact that 

 a contracted tendril acts like a spiral spring, and is thus converted into 

 a yielding, instead of an unyielding, body. The spirally-wound tendril 

 yields like an elastic thread to a pull which would break the tendril in 

 its original condition. The meaning of this arrangement is to enable 

 the plant to weather a gale which would tear it from its support by 

 snapping the tendrils, if they were not converted into spiral springs. 



My father describes how he went in a gale of wind to watch the 

 bryony on an exposed hedge, and how, in spite of the violent wind 

 which tossed the branches of the plant about, the bryony safely rode 

 out the gale, " like a ship with two anchors down, and with a long 

 range of cable ahead, to serve as a spring as she surges to the storm." 

 It may also serve to divide the weight which has to be supported 

 equally among a number of tendrils ; and this is the meaning of the 

 spiral contraction seen in the tendrils of the Virginia creeper. 



It can be seen in Fig. 4 that all the coils of the spiral are not in the 

 same direction. First, there are two in one direction, then six in the 

 other, and then three again in the first direction, making six turns in 

 one way and five in the other. And this is universally the case ; the 

 turns in one direction are always approximately equal in number to 

 those in the opposite direction. It can be shown to be a mechanical 

 necessity that a tendril which has its two ends fixed, and which then 

 coils into a spiral, should behave in this way. 



A simple model made to show this mechanical necessity is described 

 by Sachs in his "Text-book." It is made by stretching a strip of 

 India-rubber and cementing it to an unstretched strip. The strips 

 being united in a state of longitudinal strain, form a spiral when re- 

 leased. If the model is held by one end only, the turns of the spiral 

 are all in one direction. And this represents the behavior of a tendril 

 which has not managed to seize a support ; for some unknown reason 

 such tendrils contract into spirals, and the turns of such spirals are all 

 in one direction. But, if the India-rubber is held at both ends, half 

 the turns are in one direction, half in the other, just as with a tendril 

 the same thing happens. 



Now, let us consider the general relations that exist between twining 

 plants, leaf-climbing plants, and tendril-climbing plants. To an evo- 

 lutionist the question how these various classes of climbing plants have 

 been developed is perhaps of most interest. What is the relationship 

 between them ? Have all classes been developed separately from ordi- 

 nary non-climbing plants, or has one class been developed out of one 

 of the others ; and, if so, which is the oldest form of climbing plant ? 



