ELECTRIC RESPONSE IN PLANTS 23 



The spring-tapper. This consists (fig. 10) of the 

 spring proper (s), the attached rod (R) carrying at its 

 end the tapping-head (T). A projecting rod the lifter 

 (L) passes through s R. It is provided with a screw- 

 thread, by means of which its length, projecting down- 

 wards, is regulated. This fact, as we shall see, is made 

 to determine the height of the stroke, (c) is a cogwheel. 

 As one of the spokes of the cogwheel is rotated past 

 (L), the spring is lifted and released, and (T) delivers a 

 sharp tap. The height of the lift, and therefore the 

 intensity of the stroke, is measured by means of a 



R T 



FIG. 10. THE SPRING-TAPPER 



graduated scale. We can increase the intensity of 

 the stroke through a wide range (1) by increasing the 

 projecting length of the lifter, and (2) by shortening 

 the length of spring by a sliding catch. We may 

 give isolated single taps or superpose a series in rapid 

 succession according as the wheel is rotated slow or 

 fast. The only disadvantage of the tapping method of 

 stimulation is that in long-continued experiment the 

 point struck is liable to be injured. The vibrational 

 mode of stimulation to be presently described labours 

 under no such disadvantage. 



