692 . PLANT RESPONSE 



to be the quicker ; but if stimulus produce greater fatigue of 

 the lower half, \vc can see that its movement will be made 

 somewhat slower, and the upstroke will then become the 

 relatively quicker of the two. 



This anticipation finds remarkable verification in the 

 photographic record given in fig. 276. We see there, in the 

 first or normal record, from the fineness and steepness of the 

 upward line, representing the dovvnstroke, that this is the 

 quicker movement of the two ; but after the application of 

 strong stimulus of light, at the point marked with an upward 



EiG. 276. Pholographic Record of Autonomous Pulsations in Lateral 

 Leaflet of Desinodiitm gyraiis under Action of Sunlight, showing 

 I'eriodic Reversals 



Light applied continuously from arrow onwards. In the first two 

 responses, representing the normal, the downstroke of the leaflet, 

 represented by the up curve, is relatively the quicker. After the 

 application of light, this relation is gradually reversed, till in the 

 fourth and fifth pulses after application it is the upstroke, represented 

 by the down curve, which is pronouncedly the quicker. This reversal 

 is in its turn reversed at the eighth pulsation. 



arrow, we observe a gradual change, by which the existing 

 normal difference between up and down strokesis first abolished 

 and then reversed. In the next complete response, after the 

 application, we see that the two strokes have become equally 

 rapid. In the next, the downstroke has become distinctly 

 the slower, and this goes on progressively till we see in the 

 fifth of this series a very remarkable degree of difference 

 between the two, the upstroke being now much the quicker. 

 These reversals are found to be recurrent, further on in the 

 record. Such alternate changes of excitability on the two 

 sides we have noticed even in the case of radial organs ; for 



