590 PLANT RESPONSE 



figure, the suspended lever may be adjusted in any con- 

 venient azimuth. 



The free end of the growing plant is attached to one end 

 of the lever, L, being on its own level, at a distance of, say, 

 20 cm. from the line of suspension, the attached thread being 

 at right angles to the lever. There is a slight tension of the 

 thread, due to the magnetic force of the controlling magnet, 

 which tends to draw the lever away from the plant. This 

 tension, however, is very slight. When the grov/ing organ is 

 acted on by light in a horizontal direction, parallel to the 

 attached thread, and therefore at right angles to the lever, 

 then if the action of light be to induce a positive heliotropic 

 effect, the rotation of the suspended magnetic system, owing 

 to the pull exerted by the curving organ, will be, when seen 

 from the top, in the direction opposite to that of the hands of 

 a watch. 



If, however, the heliotropic effect be negative, the top of 

 the growing organ will move in the opposite direction. But 

 it has been said that there was a slight tension of the thread, 

 owing to the action of the controlling magnet. This being 

 now released, the lever will move to a proportionate extent 

 in the same direction as the hands of a watch. A spot of 

 light reflected from the mirror magnifies these movements, 

 and a record of this moving spot of light on a revolving 

 drum gives the response-curve. Instead of this magnetic 

 control, it would also be possible to use, as the controlling 

 force, the torsion of a fine metallic wire. 



By using a shorter lever arm and increasing the distance 

 of the revolving recording drum from the mirror, a wide 

 range of magnification up to 500 times may easily be 

 obtained. For ordinary purposes a magnification of ten 

 times is all that is necessary. 



The movement of the spot of light is proportionate to 

 the heliotropic movement of the plant, at least when the 

 amount of that movement is not excessive. Thus, know- 

 ing the magnification produced by the system, and the 

 rate of the revolving drum, we can determine from the 



