POSITIVE HELIOTROPISM 589 



a procedure which, as we have seen, causes disturbance. 

 The free end of a normally growing organ, when acted on, 

 say by horizontal light, bends towards it. Thus the problem 

 is to obtain a continuous time-record of this movement, from 

 which the latent period, the actual rate of movement, the 

 after-effect, and other related effects might be ascertained. 

 I have been able to solve this difficulty by devising a mag- 



i<"iG. 240. Diagrammatic Representation of the Magnetically Controlled 

 Recorder 



.l', the Optic Lever, to one arm of which the plant is attached by a 

 thread ; M, the mirror, with small magnet, Ns, attached behind. The 

 lever is rotated to dotted position by heliotropic curvature of the plant, 

 diagrammatically represented disproportionately magnified. 



netically controlled recorder, the principle of which will be 

 understood from the accompanying diagram (fig. 240). 



The principal part of this recording instrument consists 

 of a magneto-metric arrangement. Attached to a long 

 aluminium lever, ll', is a vertical T-piece, v. This T-piece, 

 V, carries a reflecting mirror, behind which is a short magnet, 

 NS. The whole arrangement is freely suspended by a silk 

 thread. By means of a controlling magnet not shown in the 



