48 MOVEMENTS OF PLANTS 



kept at right angles to the main body of the root. 

 If the theory we are testing is the right one, a root 

 with its motor region horizontal and its tip vertical 

 ought to continue to grow horizontally, because the 

 tip being vertical is not stimulated by gravity ; it 

 is in a quiescent, or, as it were, a satisfied condition, 

 and no bending influence is being sent to the motor 

 region. And this is what Pfeffer and Czapek 

 found. On the other hand, if the main body of the 

 root points vertically down while the sensitive tip 

 is horizontal, a curvature results, because as long 

 as the tip is horizontal it is stimulated, and the 

 stimulus is transmitted to the motor region. 



This experiment proves not only that the tip 

 of the root is the sense-organ for gravity, but also 

 that the motile part is not directly sensitive ; in 

 other words, that gravitation is perceived exclu- 

 sively in the tip of the root. Since the publication 

 of PfeflFer's and Czapek's papers I have been lucky 

 enough to hit on another way of investigating 

 pCTcipient organs for gravitation,^ and I am not 

 without hopes that botanists may become in this 

 question as fertile as Cyrano with his seven ways of 

 flying to the moon. 



There is a certain kind of inverted action 

 familiarly known as the tail wagging the dog, and 

 it is on this principle of inversion that my experi- 

 ment is designed. Inversion may in some cases 

 be practised without altering the final result. For 

 instance, it does not much matter whether the 

 thread goes to the eye (the rational masculine 



' F, Darwin, Annals oj Botany, December 1899. 



