IRRITABILITY OF PLANTS 141 



jewel bearings. One of the bearings is fixed at the centre 

 of a soft iron core, and the other bearing is carried by a 

 flat metallic plate. The soft iron core is surrounded by a 

 wire spiral through which flows the same current which 

 activates the reed ; so this second iron core becomes an 

 electro-magnet, and for exactly the same periods ; the reed 

 and the writer are kept in perfect unison. The bent tip 

 of the writer taps regularly upon a smoked plate, placed 

 at right angles to it. These taps must always be on the 

 same point so long as the recording surface is stationary ; 

 but if it be made to travel we shall get a row of dots, 

 made at the time-intervals predetermined. It was next 

 found most conducive to good records to let the plate descend 

 by its own weight, thus giving a vertical series of dots ; 

 for though successive distances between them are slightly 

 increasing in course of the acceleration of the falling plate, 

 this matters little for time-measurements, since their numbers 

 per second are identical. An ingenious compensatory device 

 has, however, been provided for use when required. 



The tapping method has now secured a double advan- 

 tage : (i) the precisely comparable time-records, and (2) the 

 practical elimination of friction ; since the bent tip of the 

 writer gives a series of taps, and is therefore not in continuous 

 contact with the recording surface. A fine cocoon thread is 

 securely tied to the leaf to be observed, and its other end is 

 attached to the short arm of a very light wire lever which 

 has been already fixed to the writer. The movement of 

 the leaf pulls the writer to one side or other, giving dots 

 no longer in mere vertical row, but now recording every 

 movement of the plant. The conspicuous fall of the Mimosa 

 leaf, or the minutest quiver in pulsating leaflet or of con- 

 traction under a stimulus, will thus cause a pull on the 

 attached thread ; and this will be transmitted and magni- 

 fied by the writing lever. The dots are seen to lie in 

 definite and characteristic order; and the dotted curve 

 gives the whole history of the plant-movement from start 

 to finish. 



