EFFECT OF REVERSED ANISOTROPY 243 



tension was found to occur before dawn, when the petiole 

 was erected towards the apex of the stem. Tension 

 decreased during the day and reached minimum early in 

 the evening, when the petiole fell, the movement being 

 away from the apex of the stem. The relation between 

 change of temperature and tension increased with the rise 

 and decreased with the fall of temperature. 



The anisotropy of the responding pulvinus in Mimosa is 

 natural and permanent. This suggests the question, What 

 would follow if the Mimosa plant were placed upside down ? 

 The periodic movements of the petiole in relation to the 

 axis of the plant will obviously remain the same, but will 

 appear reversed in space. Maximum tension in the morning 

 will make the petiole approach the tip of the stem — that is 

 to say, the movement will be downwards, instead of upwards 

 as in the normal position. 



Effect of Reversal of Induced Anisotropy on 



Diurnal Movement 



The next case to be considered is that in which the 

 anisotropy is not natural or permanent, but has been induced 

 by geotropic stimulation and is thus capable of becoming 

 reversed under appropriate conditions. For example, if 

 a stem, say, of Tropaeolum be held horizontal, it will curve 

 upwards ; one side, the upper (A) , will be contracted, and the 

 lower (B) expanded, the radial organ thus becoming aniso- 

 tropic. Next, if the stem be inverted by rotating it 

 through 180°, then the side A will have become the lower ; 

 the former geotropic curvature will shortly become reversed, 

 and A will undergo a change from contraction to expansion. 

 The induced anisotropy will thus undergo reversal. 



If the periodic movement depends on the anisotropy 

 that is induced by geotropic stimulation, three stages of 

 transformation should be presented in the diurnal record 

 of the plant : 



