6 PLANT RESPONSE 



tion whatever, since the movement of the leaflet is itself 

 considerable. 



A very light counterpoise is used, as will be seen later, to 

 exert a slight pull on the leaflet in an upward direction when 

 necessary, the sliding arrangement enabling us to vary the 

 amount of this tension. It will thus be seen that the leaflet is 

 practically free from constraint, and any movement, however 

 slight, is easily detected. When the leaflet falls, the spot of 

 light moves, say downwards, and vice versa. The record of 

 the entire response — down movement followed by up — may 

 thus be made on a vertical revolving drum, whose speed is 

 regulated by clock-work. The magnification of the record 

 having been determined previously, and the speed of the 

 drum being known, the response- curve gives the absolute 

 movement and the time-relations of such movement. 



Instead of using a vertical drum, it is more convenient to 

 record on the revolving surface of a horizontal drum. The 

 up and down movement of the spot of light may now be 

 converted into lateral, or left and right movement, by means 

 of a second mirror suitably inclined. The finer adjustment 

 of the reflected spot of light may be brought about by means 

 of a milled head with which the second mirror is provided. 

 A photographic record may be obtained by wrapping over 

 the drum surface a sensitised roll-film. But since these 

 movements are comparatively slow, it is easy to obtain the 

 record more simply by following the spot of light Avith a 

 recording pen, which slides on a horizontal guide-bar, parallel 

 with its movements. 



These response-records can be traced on a large scale in 

 the presence of an audience, by the use of the Demonstration 

 Recorder, which consists of a twin-drum, over which is wrapped 

 an endless band of paper to serve as the recording surface 

 (fig. 4). Elastic bands pass over the two drums, one of which 

 is kept revolving by clock-work. The excursion of the spot of 

 light is now followed by means of a sliding ink-well, from 

 which projects the ink-sponge. By this means, the tracing 

 of the response-curve, and its various modifications under the 



