13 2 ROOTS 



around an obstacle, but it does seem to be able to cause a 

 curve. It seems to be able to cause the region of elonga- 

 tion to curve, and, as a result, the tip itself is turned away 

 from the obstacle. 



Think of your fingers feeling in the dark. They may 

 touch something which gives you an unpleasant sensa- 

 tion. You snatch them away. Now the very ends of 

 your fingers which touched the object cannot move. They 

 can be moved, of course, but they cannot move themselves. 

 What they do is to start a message to your muscles which, 

 by their action, produce the movement. The fingers are 

 organs of the sense of touch. Similarly, root tips seem to 

 be organs of a sense of touch. They seem to be able to 

 start a message to which the region of curvature responds. 

 This sense of touch of plants is very delicate. It has been 

 measured by experiments upon the tips of tendrils. They 

 seem to have a sense of touch far more delicate than our 

 own. A gentle stroke of a very fine silk fiber, so gentle 

 that the end of your finger or your tongue would not per- 

 ceive it at all, is enough to make a tendril curve. 



B. Tropisms of Roots. — We have just been consider- 

 ing the way in which contact with an obstacle may affect 

 the direction of a root's growth. Other things also have 

 an effect upon the direction of its growth. Water and 

 gravity are two of these things. 



Tendencies of living things to turn in response to the 

 influence of things outside of them are called tropisms. 

 (The word tropism is derived from a Greek verb meaning 

 to turn.) You know, for example, that leaves turn to the 

 light. Tropism caused by the light is called phototropism. 

 (Recall the meaning of photosynthesis.) 



