112 BOTANY. [chap. lit. 



under the influence of light and heat alone, this move- 

 ment depending on mechanical irritation. If the tendrils 

 of the passion-flower or those of the bryony {Bryonia 

 dioica) are examined, it will be seen that in the young 

 condition they spread out from the stem either straight 

 or slightly curved, and remain for some time in this 

 condition, until the weak branch bearing them is brought 

 into contact with some neighbouring branch by the 

 wind or some other means, or in some plants the tendrils 

 themselves revolve, searching for a suitable body round 

 which they can coil. If the tendrils come in contact 

 with a flat surface or a thick branch, no change takes 

 place, but when made to touch a thin twig, the tendril 

 within a short time coils its tip round the support, and 

 then becomes rigid, so that it cannot be uncoiled without 

 breaking. This rigidity is necessary to prevent the 

 coiled portion being drawn out by the weight of its own 

 branch when moved by the wind. After the tip of the 

 tendril is firmly fixed to its support, the intermediate 

 portion undergoes a spiral contraction by which the 

 tendril is shortened. This spiral contraction of that 

 portion of the tendril between its base and tip is of value 

 from two points of view ; it gives elasticity, and conse- 

 quently greater strength, and also draws the branch 

 higher up so as to expose its leaves more fully to the 

 light. If a tendril that has become attached by the tip is 

 examined, it will be seen that one portion of the interme- 

 diate part is twisted in one direction and another portion 

 in the opposite direction, with an intermediate straight 

 part; but for this arrangement, it will be understood 

 that a body fixed at both ends and undergoing spiral 

 contraction would be ruptured. 



