506 



LIFE MOVEMENTS IN PLANTS 



of incident light. The first part of the curve is the record 

 of geotropic torsional movement. Light of a given inten- 

 sity was applied below at a point marked - L (Fig. 18L) ; 

 this is seen to produce an over-balance, the phototropic 

 effect being slightly in excess. The intensity of incident 

 light was continuously diminished by regulation of the dia- 

 phragm till an exact balance was obtained as seen in the 

 horizontal part of the record. It is with great surprise 

 that one comes to realise the fact that the effect of one 

 form of stimulus can be so exactly balanced by that of 

 another, so entirely different, and that the stimulus of 

 gravity could be measured, as it were, in candle powers of 

 light ! After securing the balance, light was cut off, 

 and the geotropic torsion became renewed on the cessation 

 of the counteracting phototropic action. ^, 



Comparative balancing effects of white and ted lights: 

 Experiment 196. — White light was at first applied at - 1, in 

 opposition to geotropic movement. The intensity of light 

 was stronger than what was necessary for exact balance, 

 and its effect was at first to retard and then reverse 

 the torsional response due to geotropism. "When thus over- 

 balanced, rexl glass was 

 interposed on the path 

 of light at R. As the 

 phototropic effect of 

 this light is feeble 

 or absent, the geo- 

 tropic torsion became 

 predominant as seen in 



Fig. 182.— Application of white light at— L iu the subsequent Up- 



opposition causes reversal of torsion. Red CUiVe. The red glaSS 



lierht U, is ineffective, and geotropic torsion in , , 



^ ' ' . ,. , was next removed sub- 



restored. Reapplication of white light causes 



unce more the reversal of torsion. stituting white light at 



- L to act once more in opposition; the result is seen in 

 the final over-balance, and reversal of torsion (Fig. 182). 



